diff --git a/toxcore/mono_time.c b/toxcore/mono_time.c
index 5130ee6d..7fdf2d7e 100644
--- a/toxcore/mono_time.c
+++ b/toxcore/mono_time.c
@@ -28,8 +28,13 @@
 struct Mono_Time {
     uint64_t time;
     uint64_t base_time;
+
+    mono_time_current_time_cb *current_time_callback;
+    void *user_data;
 };
 
+static mono_time_current_time_cb current_time_monotonic_default;
+
 Mono_Time *mono_time_new(void)
 {
     Mono_Time *mono_time = (Mono_Time *)malloc(sizeof(Mono_Time));
@@ -38,6 +43,9 @@ Mono_Time *mono_time_new(void)
         return nullptr;
     }
 
+    mono_time->current_time_callback = current_time_monotonic_default;
+    mono_time->user_data = nullptr;
+
     mono_time->time = 0;
     mono_time->base_time = (uint64_t)time(nullptr) - (current_time_monotonic(mono_time) / 1000ULL);
 
@@ -66,6 +74,24 @@ bool mono_time_is_timeout(const Mono_Time *mono_time, uint64_t timestamp, uint64
     return timestamp + timeout <= mono_time_get(mono_time);
 }
 
+void mono_time_set_current_time_callback(Mono_Time *mono_time,
+        mono_time_current_time_cb *current_time_callback, void *user_data)
+{
+    if (current_time_callback == nullptr) {
+        mono_time->current_time_callback = current_time_monotonic_default;
+        mono_time->user_data = nullptr;
+    } else {
+        mono_time->current_time_callback = current_time_callback;
+        mono_time->user_data = user_data;
+    }
+}
+
+/* return current monotonic time in milliseconds (ms). */
+uint64_t current_time_monotonic(const Mono_Time *mono_time)
+{
+    return mono_time->current_time_callback(mono_time->user_data);
+}
+
 //!TOKSTYLE-
 // No global mutable state in Tokstyle.
 #ifdef OS_WIN32
@@ -74,8 +100,7 @@ static uint64_t add_clock_mono;
 #endif
 //!TOKSTYLE+
 
-/* return current monotonic time in milliseconds (ms). */
-uint64_t current_time_monotonic(const Mono_Time *mono_time)
+static uint64_t current_time_monotonic_default(void *user_data)
 {
     uint64_t time;
 #ifdef OS_WIN32
diff --git a/toxcore/mono_time.h b/toxcore/mono_time.h
index ed69e3b0..b735f2c3 100644
--- a/toxcore/mono_time.h
+++ b/toxcore/mono_time.h
@@ -52,6 +52,16 @@ bool mono_time_is_timeout(const Mono_Time *mono_time, uint64_t timestamp, uint64
 /* return current monotonic time in milliseconds (ms). */
 uint64_t current_time_monotonic(const Mono_Time *mono_time);
 
+typedef uint64_t mono_time_current_time_cb(void *user_data);
+
+/* Override implementation of current_time_monotonic() (for tests).
+ *
+ * The caller is obligated to ensure that current_time_monotonic() continues
+ * to increase monotonically.
+ */
+void mono_time_set_current_time_callback(Mono_Time *mono_time,
+        mono_time_current_time_cb *current_time_callback, void *user_data);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/toxcore/mono_time_test.cc b/toxcore/mono_time_test.cc
index 4ed59406..6e81ac7e 100644
--- a/toxcore/mono_time_test.cc
+++ b/toxcore/mono_time_test.cc
@@ -35,4 +35,28 @@ TEST(MonoTime, IsTimeout) {
   mono_time_free(mono_time);
 }
 
+static uint64_t test_current_time_callback(void *user_data) { return *(uint64_t *)user_data; }
+
+TEST(MonoTime, CustomTime) {
+  Mono_Time *mono_time = mono_time_new();
+
+  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_update(mono_time);
+
+  EXPECT_EQ(current_time_monotonic(mono_time), test_time);
+
+  uint64_t const start = mono_time_get(mono_time);
+
+  test_time += 7000;
+
+  mono_time_update(mono_time);
+  EXPECT_EQ(mono_time_get(mono_time) - start, 7);
+
+  EXPECT_EQ(current_time_monotonic(mono_time), test_time);
+
+  mono_time_free(mono_time);
+}
+
 }  // namespace