diff --git a/auto_tests/TCP_test.c b/auto_tests/TCP_test.c
index d41739f4..1f4626db 100644
--- a/auto_tests/TCP_test.c
+++ b/auto_tests/TCP_test.c
@@ -45,7 +45,7 @@ static uint16_t ports[NUM_PORTS] = {13215, 33445, 25643};
 
 static void test_basic(void)
 {
-    Mono_Time *mono_time = mono_time_new();
+    Mono_Time *mono_time = mono_time_new(nullptr, nullptr);
     const Random *rng = system_random();
     ck_assert(rng != nullptr);
     Logger *logger = logger_new();
@@ -297,7 +297,7 @@ static int read_packet_sec_TCP(const Logger *logger, struct sec_TCP_con *con, ui
 
 static void test_some(void)
 {
-    Mono_Time *mono_time = mono_time_new();
+    Mono_Time *mono_time = mono_time_new(nullptr, nullptr);
     const Random *rng = system_random();
     ck_assert(rng != nullptr);
     Logger *logger = logger_new();
@@ -488,9 +488,9 @@ static int oob_data_callback(void *object, const uint8_t *public_key, const uint
 
 static void test_client(void)
 {
+    Mono_Time *mono_time = mono_time_new(nullptr, nullptr);
     const Random *rng = system_random();
     ck_assert(rng != nullptr);
-    Mono_Time *mono_time = mono_time_new();
     Logger *logger = logger_new();
 
     uint8_t self_public_key[CRYPTO_PUBLIC_KEY_SIZE];
@@ -619,9 +619,9 @@ static void test_client(void)
 // Test how the client handles servers that don't respond.
 static void test_client_invalid(void)
 {
+    Mono_Time *mono_time = mono_time_new(nullptr, nullptr);
     const Random *rng = system_random();
     ck_assert(rng != nullptr);
-    Mono_Time *mono_time = mono_time_new();
     Logger *logger = logger_new();
     const Network *ns = system_network();
 
@@ -694,7 +694,7 @@ static int tcp_data_callback(void *object, int id, const uint8_t *data, uint16_t
 
 static void test_tcp_connection(void)
 {
-    Mono_Time *mono_time = mono_time_new();
+    Mono_Time *mono_time = mono_time_new(nullptr, nullptr);
     Logger *logger = logger_new();
     const Random *rng = system_random();
     ck_assert(rng != nullptr);
@@ -803,7 +803,7 @@ static int tcp_oobdata_callback(void *object, const uint8_t *public_key, unsigne
 
 static void test_tcp_connection2(void)
 {
-    Mono_Time *mono_time = mono_time_new();
+    Mono_Time *mono_time = mono_time_new(nullptr, nullptr);
     Logger *logger = logger_new();
     const Random *rng = system_random();
     ck_assert(rng != nullptr);
diff --git a/auto_tests/announce_test.c b/auto_tests/announce_test.c
index 92f2d129..94b68bf1 100644
--- a/auto_tests/announce_test.c
+++ b/auto_tests/announce_test.c
@@ -57,7 +57,7 @@ static void test_store_data(void)
     Logger *log = logger_new();
     ck_assert(log != nullptr);
     logger_callback_log(log, (logger_cb *)print_debug_log, nullptr, nullptr);
-    Mono_Time *mono_time = mono_time_new();
+    Mono_Time *mono_time = mono_time_new(nullptr, nullptr);
     Networking_Core *net = new_networking_no_udp(log, ns);
     DHT *dht = new_dht(log, rng, ns, mono_time, net, true, true);
     Forwarding *forwarding = new_forwarding(log, rng, mono_time, dht);
diff --git a/auto_tests/auto_test_support.c b/auto_tests/auto_test_support.c
index 256ebb5a..3b32a286 100644
--- a/auto_tests/auto_test_support.c
+++ b/auto_tests/auto_test_support.c
@@ -144,7 +144,7 @@ void iterate_all_wait(AutoTox *autotoxes, uint32_t tox_count, uint32_t wait)
     c_sleep(5);
 }
 
-static uint64_t get_state_clock_callback(Mono_Time *mono_time, void *user_data)
+static uint64_t get_state_clock_callback(void *user_data)
 {
     const uint64_t *clock = (const uint64_t *)user_data;
     return *clock;
diff --git a/auto_tests/forwarding_test.c b/auto_tests/forwarding_test.c
index 8182d607..a352e936 100644
--- a/auto_tests/forwarding_test.c
+++ b/auto_tests/forwarding_test.c
@@ -110,7 +110,7 @@ static Forwarding_Subtox *new_forwarding_subtox(bool no_udp, uint32_t *index, ui
     subtox->log = logger_new();
     ck_assert(subtox->log != nullptr);
     logger_callback_log(subtox->log, (logger_cb *)print_debug_log, nullptr, index);
-    subtox->mono_time = mono_time_new();
+    subtox->mono_time = mono_time_new(nullptr, nullptr);
 
     const Random *rng= system_random();
     ck_assert(rng != nullptr);
diff --git a/auto_tests/lan_discovery_test.c b/auto_tests/lan_discovery_test.c
index b92fefe9..d7bf594b 100644
--- a/auto_tests/lan_discovery_test.c
+++ b/auto_tests/lan_discovery_test.c
@@ -6,7 +6,7 @@
 #include "../toxcore/tox_struct.h"
 #include "auto_test_support.h"
 
-static uint64_t get_state_clock_callback(Mono_Time *mono_time, void *user_data)
+static uint64_t get_state_clock_callback(void *user_data)
 {
     const uint64_t *clock = (const uint64_t *)user_data;
     return *clock;
diff --git a/auto_tests/onion_test.c b/auto_tests/onion_test.c
index bb6ae243..fcc65975 100644
--- a/auto_tests/onion_test.c
+++ b/auto_tests/onion_test.c
@@ -197,8 +197,8 @@ static void test_basic(void)
 
     const Random *rng = system_random();
     ck_assert(rng != nullptr);
-    Mono_Time *mono_time1 = mono_time_new();
-    Mono_Time *mono_time2 = mono_time_new();
+    Mono_Time *mono_time1 = mono_time_new(nullptr, nullptr);
+    Mono_Time *mono_time2 = mono_time_new(nullptr, nullptr);
 
     IP ip = get_loopback();
     Onion *onion1 = new_onion(log1, mono_time1, rng, new_dht(log1, rng, ns, mono_time1, new_networking(log1, ns, &ip, 36567), true, false));
@@ -293,7 +293,7 @@ static void test_basic(void)
     Logger *log3 = logger_new();
     logger_callback_log(log3, (logger_cb *)print_debug_log, nullptr, &index[2]);
 
-    Mono_Time *mono_time3 = mono_time_new();
+    Mono_Time *mono_time3 = mono_time_new(nullptr, nullptr);
 
     Onion *onion3 = new_onion(log3, mono_time3, rng, new_dht(log3, rng, ns, mono_time3, new_networking(log3, ns, &ip, 36569), true, false));
     ck_assert_msg((onion3 != nullptr), "Onion failed initializing.");
@@ -380,7 +380,7 @@ static Onions *new_onions(const Random *rng, uint16_t port, uint32_t *index)
 
     logger_callback_log(on->log, (logger_cb *)print_debug_log, nullptr, index);
 
-    on->mono_time = mono_time_new();
+    on->mono_time = mono_time_new(nullptr, nullptr);
 
     if (!on->mono_time) {
         logger_kill(on->log);
diff --git a/auto_tests/save_load_test.c b/auto_tests/save_load_test.c
index a06440e9..97077735 100644
--- a/auto_tests/save_load_test.c
+++ b/auto_tests/save_load_test.c
@@ -9,6 +9,7 @@
 #include "../testing/misc_tools.h"
 #include "../toxcore/ccompat.h"
 #include "../toxcore/tox.h"
+#include "../toxcore/tox_struct.h"
 #include "../toxcore/util.h"
 #include "auto_test_support.h"
 #include "check_compat.h"
@@ -109,7 +110,7 @@ typedef struct Time_Data {
     uint64_t clock;
 } Time_Data;
 
-static uint64_t get_state_clock_callback(Mono_Time *mono_time, void *user_data)
+static uint64_t get_state_clock_callback(void *user_data)
 {
     Time_Data *time_data = (Time_Data *)user_data;
     pthread_mutex_lock(&time_data->lock);
@@ -127,8 +128,7 @@ static void increment_clock(Time_Data *time_data, uint64_t count)
 
 static void set_current_time_callback(Tox *tox, Time_Data *time_data)
 {
-    // TODO(iphydf): Don't rely on toxcore internals.
-    Mono_Time *mono_time = ((Messenger *)tox)->mono_time;
+    Mono_Time *mono_time = tox->mono_time;
     mono_time_set_current_time_callback(mono_time, get_state_clock_callback, time_data);
 }
 
@@ -160,7 +160,7 @@ static void test_few_clients(void)
 
     Time_Data time_data;
     ck_assert_msg(pthread_mutex_init(&time_data.lock, nullptr) == 0, "Failed to init time_data mutex");
-    time_data.clock = current_time_monotonic(((Messenger *)tox1)->mono_time);
+    time_data.clock = current_time_monotonic(tox1->mono_time);
     set_current_time_callback(tox1, &time_data);
     set_current_time_callback(tox2, &time_data);
     set_current_time_callback(tox3, &time_data);
diff --git a/auto_tests/tox_events_test.c b/auto_tests/tox_events_test.c
index 4831f058..04549b21 100644
--- a/auto_tests/tox_events_test.c
+++ b/auto_tests/tox_events_test.c
@@ -9,6 +9,7 @@
 #include "../testing/misc_tools.h"
 #include "../toxcore/tox.h"
 #include "../toxcore/tox_events.h"
+#include "../toxcore/tox_struct.h"
 #include "auto_test_support.h"
 #include "check_compat.h"
 
@@ -37,7 +38,7 @@ static bool await_message(Tox **toxes)
     return false;
 }
 
-static uint64_t get_state_clock_callback(Mono_Time *mono_time, void *user_data)
+static uint64_t get_state_clock_callback(void *user_data)
 {
     const uint64_t *clock = (const uint64_t *)user_data;
     return *clock;
@@ -58,14 +59,12 @@ static void test_tox_events(void)
         ck_assert_msg(toxes[i] != nullptr, "failed to create tox instances %u", i);
     }
 
-    // TODO(iphydf): Don't rely on toxcore internals.
-    uint64_t clock = current_time_monotonic(((Messenger *)toxes[0])->mono_time);
+    uint64_t clock = current_time_monotonic(toxes[0]->mono_time);
     Mono_Time *mono_time;
 
-    // TODO(iphydf): Don't rely on toxcore internals.
-    mono_time = ((Messenger *)toxes[0])->mono_time;
+    mono_time = toxes[0]->mono_time;
     mono_time_set_current_time_callback(mono_time, get_state_clock_callback, &clock);
-    mono_time = ((Messenger *)toxes[1])->mono_time;
+    mono_time = toxes[1]->mono_time;
     mono_time_set_current_time_callback(mono_time, get_state_clock_callback, &clock);
 
     uint8_t pk[TOX_PUBLIC_KEY_SIZE];
diff --git a/auto_tests/toxav_many_test.c b/auto_tests/toxav_many_test.c
index 86420509..725fe613 100644
--- a/auto_tests/toxav_many_test.c
+++ b/auto_tests/toxav_many_test.c
@@ -16,6 +16,7 @@
 #include "../toxcore/crypto_core.h"
 #include "../toxcore/logger.h"
 #include "../toxcore/tox.h"
+#include "../toxcore/tox_struct.h"
 #include "../toxcore/util.h"
 #include "auto_test_support.h"
 #include "check_compat.h"
@@ -134,7 +135,7 @@ typedef struct Time_Data {
     uint64_t clock;
 } Time_Data;
 
-static uint64_t get_state_clock_callback(Mono_Time *mono_time, void *user_data)
+static uint64_t get_state_clock_callback(void *user_data)
 {
     Time_Data *time_data = (Time_Data *)user_data;
     pthread_mutex_lock(&time_data->lock);
@@ -152,8 +153,7 @@ static void increment_clock(Time_Data *time_data, uint64_t count)
 
 static void set_current_time_callback(Tox *tox, Time_Data *time_data)
 {
-    // TODO(iphydf): Don't rely on toxcore internals.
-    Mono_Time *mono_time = ((Messenger *)tox)->mono_time;
+    Mono_Time *mono_time = tox->mono_time;
     mono_time_set_current_time_callback(mono_time, get_state_clock_callback, time_data);
 }
 
@@ -174,8 +174,7 @@ static void test_av_three_calls(void)
         bootstrap = tox_new_log(nullptr, &error, &index[0]);
         ck_assert(error == TOX_ERR_NEW_OK);
 
-        // TODO(iphydf): Don't rely on toxcore internals.
-        time_data.clock = current_time_monotonic(((Messenger *)bootstrap)->mono_time);
+        time_data.clock = current_time_monotonic(bootstrap->mono_time);
         set_current_time_callback(bootstrap, &time_data);
 
         Alice = tox_new_log(nullptr, &error, &index[1]);
diff --git a/other/DHT_bootstrap.c b/other/DHT_bootstrap.c
index 1283b97b..56796ada 100644
--- a/other/DHT_bootstrap.c
+++ b/other/DHT_bootstrap.c
@@ -142,8 +142,8 @@ int main(int argc, char *argv[])
         logger_callback_log(logger, print_log, nullptr, nullptr);
     }
 
-    Mono_Time *mono_time = mono_time_new();
     const Random *rng = system_random();
+    Mono_Time *mono_time = mono_time_new(nullptr, nullptr);
     const uint16_t start_port = PORT;
     const uint16_t end_port = start_port + (TOX_PORTRANGE_TO - TOX_PORTRANGE_FROM);
     const Network *ns = system_network();
diff --git a/other/bootstrap_daemon/docker/tox-bootstrapd.sha256 b/other/bootstrap_daemon/docker/tox-bootstrapd.sha256
index 36581325..7d90b8a8 100644
--- a/other/bootstrap_daemon/docker/tox-bootstrapd.sha256
+++ b/other/bootstrap_daemon/docker/tox-bootstrapd.sha256
@@ -1 +1 @@
-cfa2c61f92df59419af678ef9d6fcd4f5609a505235d77a54c1a4ec66f1c10af  /usr/local/bin/tox-bootstrapd
+5796f74532156d61d5c715914b5013ae0544892f7d6dca237b1f6a828f49fc9c  /usr/local/bin/tox-bootstrapd
diff --git a/other/bootstrap_daemon/src/tox-bootstrapd.c b/other/bootstrap_daemon/src/tox-bootstrapd.c
index e9634216..735bf0c2 100644
--- a/other/bootstrap_daemon/src/tox-bootstrapd.c
+++ b/other/bootstrap_daemon/src/tox-bootstrapd.c
@@ -307,7 +307,7 @@ int main(int argc, char *argv[])
         }
     }
 
-    Mono_Time *const mono_time = mono_time_new();
+    Mono_Time *const mono_time = mono_time_new(nullptr, nullptr);
 
     if (mono_time == nullptr) {
         log_write(LOG_LEVEL_ERROR, "Couldn't initialize monotonic timer. Exiting.\n");
diff --git a/testing/Messenger_test.c b/testing/Messenger_test.c
index 0fd4aa32..b36d2a15 100644
--- a/testing/Messenger_test.c
+++ b/testing/Messenger_test.c
@@ -92,7 +92,7 @@ int main(int argc, char *argv[])
         exit(0);
     }
 
-    Mono_Time *const mono_time = mono_time_new();
+    Mono_Time *const mono_time = mono_time_new(nullptr, nullptr);
 
     if (mono_time == nullptr) {
         fputs("Failed to allocate monotonic timer datastructure\n", stderr);
diff --git a/testing/fuzzing/BUILD.bazel b/testing/fuzzing/BUILD.bazel
index 254c58e8..87cd5725 100644
--- a/testing/fuzzing/BUILD.bazel
+++ b/testing/fuzzing/BUILD.bazel
@@ -13,6 +13,7 @@ cc_library(
     srcs = ["fuzz_support.cc"],
     hdrs = ["fuzz_support.h"],
     visibility = ["//c-toxcore:__subpackages__"],
+    deps = ["//c-toxcore/toxcore:tox"],
 )
 
 cc_library(
@@ -27,7 +28,10 @@ cc_fuzz_test(
     srcs = ["bootstrap_harness.cc"],
     copts = ["-UNDEBUG"],
     corpus = ["//tools/toktok-fuzzer/corpus:bootstrap_fuzzer"],
-    deps = ["//c-toxcore/toxcore:tox"],
+    deps = [
+        ":fuzz_support",
+        "//c-toxcore/toxcore:tox",
+    ],
 )
 
 cc_fuzz_test(
@@ -35,5 +39,8 @@ cc_fuzz_test(
     srcs = ["toxsave_harness.cc"],
     copts = ["-UNDEBUG"],
     corpus = ["//tools/toktok-fuzzer/corpus:toxsave_fuzzer"],
-    deps = ["//c-toxcore/toxcore:tox"],
+    deps = [
+        ":fuzz_support",
+        "//c-toxcore/toxcore:tox",
+    ],
 )
diff --git a/testing/fuzzing/bootstrap_harness.cc b/testing/fuzzing/bootstrap_harness.cc
index dfcf1d17..49869e29 100644
--- a/testing/fuzzing/bootstrap_harness.cc
+++ b/testing/fuzzing/bootstrap_harness.cc
@@ -1,27 +1,34 @@
 #include <cassert>
 #include <cstring>
+#include <memory>
 
 #include "../../toxcore/tox.h"
-#include "../../toxcore/tox_struct.h"
+#include "../../toxcore/tox_private.h"
 #include "fuzz_adapter.h"
+#include "fuzz_support.h"
 
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size);
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
 {
     network_adapter_init(data, size);
 
-    Tox_Err_New error_new;
-    Tox *tox = tox_new(nullptr, &error_new);
-
     uint64_t clock = 0;
-    mono_time_set_current_time_callback(
-        tox->mono_time,
-        [](Mono_Time *mono_time, void *user_data) { return *static_cast<uint64_t *>(user_data); },
-        &clock);
+    auto sys = fuzz_system(clock);
+    assert(sys->mono_time_callback != nullptr);
+    assert(sys->mono_time_user_data != nullptr);
+
+    Tox_Options *opts = tox_options_new(nullptr);
+    assert(opts != nullptr);
+    tox_options_set_operating_system(opts, sys.get());
+
+    Tox_Err_New error_new;
+    Tox *tox = tox_new(opts, &error_new);
 
     assert(tox != nullptr);
     assert(error_new == TOX_ERR_NEW_OK);
 
+    tox_options_free(opts);
+
     uint8_t pub_key[TOX_PUBLIC_KEY_SIZE] = {0};
 
     const bool success = tox_bootstrap(tox, "127.0.0.1", 12345, pub_key, nullptr);
diff --git a/testing/fuzzing/fuzz_support.cc b/testing/fuzzing/fuzz_support.cc
index dc568801..b3e8ed32 100644
--- a/testing/fuzzing/fuzz_support.cc
+++ b/testing/fuzzing/fuzz_support.cc
@@ -3,3 +3,19 @@
  */
 
 #include "fuzz_support.h"
+
+#include <memory>
+
+#include "../../toxcore/crypto_core.h"
+#include "../../toxcore/network.h"
+#include "../../toxcore/tox_private.h"
+
+std::unique_ptr<Tox_System> fuzz_system(uint64_t &clock)
+{
+    auto sys = std::make_unique<Tox_System>();
+    sys->mono_time_callback = [](void *user_data) { return *static_cast<uint64_t *>(user_data); };
+    sys->mono_time_user_data = &clock;
+    sys->rng = system_random();  // TODO(iphydf): Put fuzz_random here.
+    sys->ns = system_network();  // TODO(iphydf): Put fuzz_network here.
+    return sys;
+}
diff --git a/testing/fuzzing/fuzz_support.h b/testing/fuzzing/fuzz_support.h
index c751a86b..a8a1d8da 100644
--- a/testing/fuzzing/fuzz_support.h
+++ b/testing/fuzzing/fuzz_support.h
@@ -7,8 +7,11 @@
 
 #include <cstdint>
 #include <cstdlib>
+#include <memory>
 #include <utility>
 
+#include "../../toxcore/tox.h"
+
 struct Fuzz_Data {
     const uint8_t *data;
     std::size_t size;
@@ -94,4 +97,6 @@ void fuzz_select_target(const uint8_t *data, std::size_t size, Args &&... args)
     return fuzz_select_target(selector, input, std::forward<Args>(args)...);
 }
 
+std::unique_ptr<Tox_System> fuzz_system(uint64_t &clock);
+
 #endif  // C_TOXCORE_TESTING_FUZZING_FUZZ_SUPPORT_H
diff --git a/testing/fuzzing/toxsave_harness.cc b/testing/fuzzing/toxsave_harness.cc
index 7005b3f7..068c9d9a 100644
--- a/testing/fuzzing/toxsave_harness.cc
+++ b/testing/fuzzing/toxsave_harness.cc
@@ -3,6 +3,8 @@
 #include <vector>
 
 #include "../../toxcore/tox.h"
+#include "../../toxcore/tox_private.h"
+#include "fuzz_support.h"
 
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size);
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
@@ -14,6 +16,10 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
     assert(tox_options != nullptr);
     assert(error_options == TOX_ERR_OPTIONS_NEW_OK);
 
+    uint64_t clock = 0;
+    auto sys = fuzz_system(clock);
+    tox_options_set_operating_system(tox_options, sys.get());
+
     // pass test data to Tox
     tox_options_set_savedata_data(tox_options, data, size);
     tox_options_set_savedata_type(tox_options, TOX_SAVEDATA_TYPE_TOX_SAVE);
diff --git a/toxav/toxav.c b/toxav/toxav.c
index ef5f7b5f..4b14aa83 100644
--- a/toxav/toxav.c
+++ b/toxav/toxav.c
@@ -181,7 +181,7 @@ ToxAV *toxav_new(Tox *tox, Toxav_Err_New *error)
 
     av->tox = tox;
     av->m = m;
-    av->toxav_mono_time = mono_time_new();
+    av->toxav_mono_time = mono_time_new(nullptr, nullptr);
     av->msi = msi_new(av->m);
 
     if (av->msi == nullptr) {
diff --git a/toxcore/mono_time.c b/toxcore/mono_time.c
index eac3f686..176dbf17 100644
--- a/toxcore/mono_time.c
+++ b/toxcore/mono_time.c
@@ -52,9 +52,11 @@ struct Mono_Time {
 };
 
 #ifdef OS_WIN32
-non_null(1) nullable(2)
-static uint64_t current_time_monotonic_default(Mono_Time *mono_time, void *user_data)
+non_null()
+static uint64_t current_time_monotonic_default(void *user_data)
 {
+    Mono_Time *const mono_time = (Mono_Time *)user_data;
+
     /* Must hold mono_time->last_clock_lock here */
 
     /* GetTickCount provides only a 32 bit counter, but we can't use
@@ -86,8 +88,8 @@ static uint64_t timespec_to_u64(struct timespec clock_mono)
     return 1000ULL * clock_mono.tv_sec + (clock_mono.tv_nsec / 1000000ULL);
 }
 #ifdef __APPLE__
-non_null(1) nullable(2)
-static uint64_t current_time_monotonic_default(Mono_Time *mono_time, void *user_data)
+non_null()
+static uint64_t current_time_monotonic_default(void *user_data)
 {
     struct timespec clock_mono;
     clock_serv_t muhclock;
@@ -102,13 +104,13 @@ static uint64_t current_time_monotonic_default(Mono_Time *mono_time, void *user_
     return timespec_to_u64(clock_mono);
 }
 #else // !__APPLE__
-non_null(1) nullable(2)
-static uint64_t current_time_monotonic_default(Mono_Time *mono_time, void *user_data)
+non_null()
+static uint64_t current_time_monotonic_default(void *user_data)
 {
 #ifdef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
     // This assert should always fail. If it does, the fuzzing harness didn't
     // override the mono time callback.
-    assert(mono_time == nullptr);
+    assert(user_data == nullptr);
 #endif
     struct timespec clock_mono;
     clock_gettime(CLOCK_MONOTONIC, &clock_mono);
@@ -118,7 +120,7 @@ static uint64_t current_time_monotonic_default(Mono_Time *mono_time, void *user_
 #endif // !OS_WIN32
 
 
-Mono_Time *mono_time_new(void)
+Mono_Time *mono_time_new(mono_time_current_time_cb *current_time_callback, void *user_data)
 {
     Mono_Time *mono_time = (Mono_Time *)calloc(1, sizeof(Mono_Time));
 
@@ -139,8 +141,7 @@ Mono_Time *mono_time_new(void)
         return nullptr;
     }
 
-    mono_time->current_time_callback = current_time_monotonic_default;
-    mono_time->user_data = nullptr;
+    mono_time_set_current_time_callback(mono_time, current_time_callback, user_data);
 
 #ifdef OS_WIN32
 
@@ -157,16 +158,14 @@ Mono_Time *mono_time_new(void)
 
     mono_time->cur_time = 0;
 #ifdef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
-    // Maximum reproducibility and don't update mono_time before the harness has
-    // had a chance to set the callback.
-    // TODO(iphydf): Put mono time callback into Tox_Options with accessors only
-    // in tox_private.h.
-    mono_time->base_time = 0;
+    // Maximum reproducibility. Never return time = 0.
+    mono_time->base_time = 1;
 #else
     mono_time->base_time = (uint64_t)time(nullptr) - (current_time_monotonic(mono_time) / 1000ULL);
-    mono_time_update(mono_time);
 #endif
 
+    mono_time_update(mono_time);
+
     return mono_time;
 }
 
@@ -191,7 +190,7 @@ void mono_time_update(Mono_Time *mono_time)
     pthread_mutex_lock(&mono_time->last_clock_lock);
     mono_time->last_clock_update = true;
 #endif
-    cur_time = mono_time->current_time_callback(mono_time, mono_time->user_data) / 1000ULL;
+    cur_time = mono_time->current_time_callback(mono_time->user_data) / 1000ULL;
     cur_time += mono_time->base_time;
 #ifdef OS_WIN32
     pthread_mutex_unlock(&mono_time->last_clock_lock);
@@ -225,7 +224,7 @@ void mono_time_set_current_time_callback(Mono_Time *mono_time,
 {
     if (current_time_callback == nullptr) {
         mono_time->current_time_callback = current_time_monotonic_default;
-        mono_time->user_data = nullptr;
+        mono_time->user_data = mono_time;
     } else {
         mono_time->current_time_callback = current_time_callback;
         mono_time->user_data = user_data;
@@ -244,7 +243,7 @@ uint64_t current_time_monotonic(Mono_Time *mono_time)
      * but must protect against other threads */
     pthread_mutex_lock(&mono_time->last_clock_lock);
 #endif
-    const uint64_t cur_time = mono_time->current_time_callback(mono_time, mono_time->user_data);
+    const uint64_t cur_time = mono_time->current_time_callback(mono_time->user_data);
 #ifdef OS_WIN32
     pthread_mutex_unlock(&mono_time->last_clock_lock);
 #endif
diff --git a/toxcore/mono_time.h b/toxcore/mono_time.h
index 9336ee16..5a367247 100644
--- a/toxcore/mono_time.h
+++ b/toxcore/mono_time.h
@@ -45,7 +45,10 @@ extern "C" {
  */
 typedef struct Mono_Time Mono_Time;
 
-Mono_Time *mono_time_new(void);
+typedef uint64_t mono_time_current_time_cb(void *user_data);
+
+nullable(1, 2)
+Mono_Time *mono_time_new(mono_time_current_time_cb *current_time_callback, void *user_data);
 
 nullable(1)
 void mono_time_free(Mono_Time *mono_time);
@@ -76,8 +79,6 @@ bool mono_time_is_timeout(const Mono_Time *mono_time, uint64_t timestamp, uint64
 non_null()
 uint64_t current_time_monotonic(Mono_Time *mono_time);
 
-typedef uint64_t mono_time_current_time_cb(Mono_Time *mono_time, void *user_data);
-
 /**
  * Override implementation of `current_time_monotonic()` (for tests).
  *
diff --git a/toxcore/mono_time_test.cc b/toxcore/mono_time_test.cc
index 87216c4a..08ac41f3 100644
--- a/toxcore/mono_time_test.cc
+++ b/toxcore/mono_time_test.cc
@@ -6,7 +6,7 @@ namespace {
 
 TEST(MonoTime, UnixTimeIncreasesOverTime)
 {
-    Mono_Time *mono_time = mono_time_new();
+    Mono_Time *mono_time = mono_time_new(nullptr, nullptr);
     ASSERT_NE(mono_time, nullptr);
 
     mono_time_update(mono_time);
@@ -24,7 +24,7 @@ TEST(MonoTime, UnixTimeIncreasesOverTime)
 
 TEST(MonoTime, IsTimeout)
 {
-    Mono_Time *mono_time = mono_time_new();
+    Mono_Time *mono_time = mono_time_new(nullptr, nullptr);
     ASSERT_NE(mono_time, nullptr);
 
     uint64_t const start = mono_time_get(mono_time);
@@ -39,19 +39,15 @@ TEST(MonoTime, IsTimeout)
     mono_time_free(mono_time);
 }
 
-uint64_t test_current_time_callback(Mono_Time *mono_time, void *user_data)
-{
-    return *static_cast<uint64_t *>(user_data);
-}
-
 TEST(MonoTime, CustomTime)
 {
-    Mono_Time *mono_time = mono_time_new();
+    Mono_Time *mono_time = mono_time_new(nullptr, nullptr);
     ASSERT_NE(mono_time, nullptr);
 
     uint64_t test_time = current_time_monotonic(mono_time) + 42137;
 
-    mono_time_set_current_time_callback(mono_time, test_current_time_callback, &test_time);
+    mono_time_set_current_time_callback(
+        mono_time, [](void *user_data) { return *static_cast<uint64_t *>(user_data); }, &test_time);
     mono_time_update(mono_time);
 
     EXPECT_EQ(current_time_monotonic(mono_time), test_time);
diff --git a/toxcore/ping_array_test.cc b/toxcore/ping_array_test.cc
index 2ac76ddd..36441031 100644
--- a/toxcore/ping_array_test.cc
+++ b/toxcore/ping_array_test.cc
@@ -49,10 +49,10 @@ TEST(PingArray, ArraySizeMustBePowerOfTwo)
 TEST(PingArray, StoredDataCanBeRetrieved)
 {
     Ping_Array_Ptr const arr(ping_array_new(2, 1));
-    Mono_Time_Ptr const mono_time(mono_time_new());
+    Mono_Time_Ptr const mono_time(mono_time_new(nullptr, nullptr));
+    ASSERT_NE(mono_time, nullptr);
     const Random *rng = system_random();
     ASSERT_NE(rng, nullptr);
-    ASSERT_NE(mono_time, nullptr);
 
     uint64_t const ping_id = ping_array_add(
         arr.get(), mono_time.get(), rng, std::vector<uint8_t>{1, 2, 3, 4}.data(), 4);
@@ -66,10 +66,10 @@ TEST(PingArray, StoredDataCanBeRetrieved)
 TEST(PingArray, RetrievingDataWithTooSmallOutputBufferHasNoEffect)
 {
     Ping_Array_Ptr const arr(ping_array_new(2, 1));
-    Mono_Time_Ptr const mono_time(mono_time_new());
+    Mono_Time_Ptr const mono_time(mono_time_new(nullptr, nullptr));
+    ASSERT_NE(mono_time, nullptr);
     const Random *rng = system_random();
     ASSERT_NE(rng, nullptr);
-    ASSERT_NE(mono_time, nullptr);
 
     uint64_t const ping_id = ping_array_add(
         arr.get(), mono_time.get(), rng, (std::vector<uint8_t>{1, 2, 3, 4}).data(), 4);
@@ -87,10 +87,10 @@ TEST(PingArray, RetrievingDataWithTooSmallOutputBufferHasNoEffect)
 TEST(PingArray, ZeroLengthDataCanBeAdded)
 {
     Ping_Array_Ptr const arr(ping_array_new(2, 1));
-    Mono_Time_Ptr const mono_time(mono_time_new());
+    Mono_Time_Ptr const mono_time(mono_time_new(nullptr, nullptr));
+    ASSERT_NE(mono_time, nullptr);
     const Random *rng = system_random();
     ASSERT_NE(rng, nullptr);
-    ASSERT_NE(mono_time, nullptr);
 
     uint8_t c = 0;
     uint64_t const ping_id = ping_array_add(arr.get(), mono_time.get(), rng, &c, sizeof(c));
@@ -102,7 +102,7 @@ TEST(PingArray, ZeroLengthDataCanBeAdded)
 TEST(PingArray, PingId0IsInvalid)
 {
     Ping_Array_Ptr const arr(ping_array_new(2, 1));
-    Mono_Time_Ptr const mono_time(mono_time_new());
+    Mono_Time_Ptr const mono_time(mono_time_new(nullptr, nullptr));
     ASSERT_NE(mono_time, nullptr);
 
     uint8_t c = 0;
@@ -113,10 +113,10 @@ TEST(PingArray, PingId0IsInvalid)
 TEST(PingArray, DataCanOnlyBeRetrievedOnce)
 {
     Ping_Array_Ptr const arr(ping_array_new(2, 1));
-    Mono_Time_Ptr const mono_time(mono_time_new());
+    Mono_Time_Ptr const mono_time(mono_time_new(nullptr, nullptr));
+    ASSERT_NE(mono_time, nullptr);
     const Random *rng = system_random();
     ASSERT_NE(rng, nullptr);
-    ASSERT_NE(mono_time, nullptr);
 
     uint8_t c = 0;
     uint64_t const ping_id = ping_array_add(arr.get(), mono_time.get(), rng, &c, sizeof(c));
@@ -129,10 +129,10 @@ TEST(PingArray, DataCanOnlyBeRetrievedOnce)
 TEST(PingArray, PingIdMustMatchOnCheck)
 {
     Ping_Array_Ptr const arr(ping_array_new(1, 1));
-    Mono_Time_Ptr const mono_time(mono_time_new());
+    Mono_Time_Ptr const mono_time(mono_time_new(nullptr, nullptr));
+    ASSERT_NE(mono_time, nullptr);
     const Random *rng = system_random();
     ASSERT_NE(rng, nullptr);
-    ASSERT_NE(mono_time, nullptr);
 
     uint8_t c = 0;
     uint64_t const ping_id = ping_array_add(arr.get(), mono_time.get(), rng, &c, sizeof(c));
diff --git a/toxcore/tox.c b/toxcore/tox.c
index e6608d75..079f4040 100644
--- a/toxcore/tox.c
+++ b/toxcore/tox.c
@@ -520,7 +520,22 @@ Tox *tox_new(const struct Tox_Options *options, Tox_Err_New *error)
         }
     }
 
-    tox_set_network(tox, nullptr);
+    const Tox_System *sys = tox_options_get_operating_system(opts);
+    const Tox_System default_system = tox_default_system();
+    if (sys == nullptr) {
+        sys = &default_system;
+    }
+
+    if (sys->rng == nullptr || sys->ns == nullptr) {
+        // TODO(iphydf): Not quite right, but similar.
+        SET_ERROR_PARAMETER(error, TOX_ERR_NEW_MALLOC);
+        tox_options_free(default_options);
+        free(tox);
+        return nullptr;
+    }
+
+    tox->rng = *sys->rng;
+    tox->ns = *sys->ns;
 
     if (m_options.proxy_info.proxy_type != TCP_PROXY_NONE) {
         if (tox_options_get_proxy_port(opts) == 0) {
@@ -549,18 +564,7 @@ Tox *tox_new(const struct Tox_Options *options, Tox_Err_New *error)
         m_options.proxy_info.ip_port.port = net_htons(tox_options_get_proxy_port(opts));
     }
 
-    const Random *rng = system_random();
-
-    if (rng == nullptr) {
-        // TODO(iphydf): Not quite right, but similar.
-        SET_ERROR_PARAMETER(error, TOX_ERR_NEW_MALLOC);
-        tox_options_free(default_options);
-        free(tox);
-        return nullptr;
-    }
-
-    tox->rng = *rng;
-    tox->mono_time = mono_time_new();
+    tox->mono_time = mono_time_new(sys->mono_time_callback, sys->mono_time_user_data);
 
     if (tox->mono_time == nullptr) {
         SET_ERROR_PARAMETER(error, TOX_ERR_NEW_MALLOC);
@@ -2576,13 +2580,3 @@ uint16_t tox_self_get_tcp_port(const Tox *tox, Tox_Err_Get_Port *error)
     tox_unlock(tox);
     return 0;
 }
-
-void tox_set_network(Tox *tox, const Network *ns)
-{
-    assert(tox != nullptr);
-    if (ns != nullptr) {
-        tox->ns = *ns;
-    } else {
-        tox->ns = *system_network();
-    }
-}
diff --git a/toxcore/tox.h b/toxcore/tox.h
index 4cd0dea6..73ce452c 100644
--- a/toxcore/tox.h
+++ b/toxcore/tox.h
@@ -501,6 +501,16 @@ typedef void tox_log_cb(Tox *tox, Tox_Log_Level level, const char *file, uint32_
                         const char *message, void *user_data);
 
 
+/**
+ * @brief Operating system functions used by Tox.
+ *
+ * This struct is opaque and generally shouldn't be used in clients, but in
+ * combination with tox_private.h, it allows tests to inject non-IO (hermetic)
+ * versions of low level network, RNG, and time keeping functions.
+ */
+typedef struct Tox_System Tox_System;
+
+
 /**
  * @brief This struct contains all the startup options for Tox.
  *
@@ -673,6 +683,12 @@ struct Tox_Options {
      */
     bool experimental_thread_safety;
 
+    /**
+     * Low level operating system functionality such as send/recv and random
+     * number generation.
+     */
+    const Tox_System *operating_system;
+
 };
 
 
@@ -744,6 +760,10 @@ bool tox_options_get_experimental_thread_safety(const struct Tox_Options *option
 
 void tox_options_set_experimental_thread_safety(struct Tox_Options *options, bool experimental_thread_safety);
 
+const Tox_System *tox_options_get_operating_system(const struct Tox_Options *options);
+
+void tox_options_set_operating_system(struct Tox_Options *options, const Tox_System *operating_system);
+
 /**
  * @brief Initialises a Tox_Options object with the default options.
  *
diff --git a/toxcore/tox_api.c b/toxcore/tox_api.c
index c74b4f6a..00051e49 100644
--- a/toxcore/tox_api.c
+++ b/toxcore/tox_api.c
@@ -97,7 +97,6 @@ uint32_t tox_dht_node_public_key_size(void)
     return TOX_DHT_NODE_PUBLIC_KEY_SIZE;
 }
 
-
 //!TOKSTYLE-
 
 #define ACCESSORS(type, ns, name) \
@@ -126,6 +125,7 @@ ACCESSORS(void *, log_, user_data)
 ACCESSORS(bool,, local_discovery_enabled)
 ACCESSORS(bool,, dht_announcements_enabled)
 ACCESSORS(bool,, experimental_thread_safety)
+ACCESSORS(const Tox_System *,, operating_system)
 
 //!TOKSTYLE+
 
diff --git a/toxcore/tox_private.c b/toxcore/tox_private.c
index cc171f1c..847e96d4 100644
--- a/toxcore/tox_private.c
+++ b/toxcore/tox_private.c
@@ -21,6 +21,17 @@
         }                             \
     } while (0)
 
+Tox_System tox_default_system(void)
+{
+    const Tox_System sys = {
+        nullptr,  // mono_time_callback
+        nullptr,  // mono_time_user_data
+        system_random(),
+        system_network(),
+    };
+    return sys;
+}
+
 void tox_lock(const Tox *tox)
 {
     if (tox->mutex != nullptr) {
diff --git a/toxcore/tox_private.h b/toxcore/tox_private.h
index 714a6a3c..71d7a976 100644
--- a/toxcore/tox_private.h
+++ b/toxcore/tox_private.h
@@ -10,14 +10,23 @@
 #include <stddef.h>
 #include <stdint.h>
 
-#include "DHT.h"
-#include "network.h"
 #include "tox.h"
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
+typedef uint64_t tox_mono_time_cb(void *user_data);
+
+struct Tox_System {
+    tox_mono_time_cb *mono_time_callback;
+    void *mono_time_user_data;
+    const struct Random *rng;
+    const struct Network *ns;
+};
+
+Tox_System tox_default_system(void);
+
 void tox_lock(const Tox *tox);
 void tox_unlock(const Tox *tox);
 
@@ -130,8 +139,6 @@ typedef enum Tox_Err_Dht_Get_Nodes {
 bool tox_dht_get_nodes(const Tox *tox, const uint8_t *public_key, const char *ip, uint16_t port,
                        const uint8_t *target_public_key, Tox_Err_Dht_Get_Nodes *error);
 
-void tox_set_network(Tox *tox, const Network *ns);
-
 #ifdef __cplusplus
 }
 #endif