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diff --git a/src/core/timer_gettimeofday.c b/src/core/timer_gettimeofday.c
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+/*
+ * (C) 2016 by sysmocom - s.f.m.c. GmbH <info@sysmocom.de>
+ * Authors: Neels Hofmeyr <nhofmeyr@sysmocom.de>
+ * All Rights Reserved
+ *
+ * SPDX-License-Identifier: GPL-2.0+
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ */
+
+/*! \addtogroup timer
+ *  @{
+ * \file timer_gettimeofday.c
+ * Overriding Time: osmo_gettimeofday()
+ *      - Useful to write and reproduce tests that depend on specific time
+ *        factors. This API allows to fake the timeval provided by `gettimeofday()`
+ *        by using a small shim osmo_gettimeofday().
+ *      - If the clock override is disabled (default) for a given clock,
+ *        osmo_gettimeofday() will do the same as regular `gettimeofday()`.
+ *      - If you want osmo_gettimeofday() to provide a specific time, you must
+ *        enable time override by setting the global variable
+ *        osmo_gettimeofday_override (`osmo_gettimeofday_override = true`), then
+ *        set the global struct timeval osmo_gettimeofday_override_time wih the
+ *        desired value. Next time osmo_gettimeofday() is called, it will return
+ *        the values previously set.
+ *      - A helper osmo_gettimeofday_override_add() is provided to easily
+ *        increment osmo_gettimeofday_override_time with a specific amount of
+ *        time.
+ */
+
+#include <stdbool.h>
+#include <sys/time.h>
+#include <osmocom/core/timer_compat.h>
+
+bool osmo_gettimeofday_override = false;
+struct timeval osmo_gettimeofday_override_time = { 23, 424242 };
+
+/*! shim around gettimeofday to be able to set the time manually.
+ * To override, set osmo_gettimeofday_override == true and set the desired
+ * current time in osmo_gettimeofday_override_time.
+ *
+ * N. B: gettimeofday() is affected by discontinuous jumps in the system time
+ *       (e.g., if the system administrator manually changes the system time).
+ *       Hence this should NEVER be used for elapsed time computation.
+ *       Instead, osmo_clock_gettime() with CLOCK_MONOTONIC should be used for that.
+ */
+int osmo_gettimeofday(struct timeval *tv, struct timezone *tz)
+{
+	if (osmo_gettimeofday_override) {
+		*tv = osmo_gettimeofday_override_time;
+		return 0;
+	}
+
+	return gettimeofday(tv, tz);
+}
+
+/*! convenience function to advance the fake time.
+ * Add the given values to osmo_gettimeofday_override_time. */
+void osmo_gettimeofday_override_add(time_t secs, suseconds_t usecs)
+{
+	struct timeval val = { secs, usecs };
+	timeradd(&osmo_gettimeofday_override_time, &val,
+		 &osmo_gettimeofday_override_time);
+}
+
+/*! @} */