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diff --git a/src/time/SystemClock.hxx b/src/time/SystemClock.hxx
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+/*
+ * Copyright 2021 Max Kellermann <max.kellermann@gmail.com>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the
+ * distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
+ * FOUNDATION OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+ * OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <chrono>
+
+/**
+ * Calculate a std::chrono::system_clock::time_point from a duration
+ * relative to the UNIX epoch (1970-01-01T00:00Z).
+ */
+template<class Rep, class Period>
+[[gnu::const]]
+std::chrono::system_clock::time_point
+TimePointAfterUnixEpoch(const std::chrono::duration<Rep,Period> &d) noexcept
+{
+	/* this is guaranteed to be 0 in C++20 */
+	const auto unix_epoch = std::chrono::system_clock::from_time_t(0);
+
+	return unix_epoch +
+		std::chrono::duration_cast<std::chrono::system_clock::duration>(d);
+}
+
+/**
+ * Calculate the duration that has passed since the UNIX epoch
+ * (1970-01-01T00:00Z).
+ */
+[[gnu::const]]
+inline std::chrono::system_clock::duration
+DurationSinceUnixEpoch(const std::chrono::system_clock::time_point &tp) noexcept
+{
+	/* this is guaranteed to be 0 in C++20 */
+	const auto unix_epoch = std::chrono::system_clock::from_time_t(0);
+
+	return tp - unix_epoch;
+}