--- a/Makefile	Mon Jan 06 21:14:35 2020 +0100
+++ b/Makefile	Mon Jan 06 21:27:45 2020 +0100
@@ -21,7 +21,8 @@
 CC=             clang
 CFLAGS=         -O3 -DNDEBUG -std=c99 -Wall -Wextra
 CPPFLAGS=       -MMD
-SRCS=           src/image.c \
+SRCS=           src/clock.c \
+                src/image.c \
                 src/main.c \
                 src/sprite.c \
                 src/texture.c \

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/clock.c	Mon Jan 06 21:27:45 2020 +0100
@@ -0,0 +1,33 @@
+/*
+ * clock.c -- track elapsed time
+ *
+ * Copyright (c) 2020 David Demelier <markand@malikania.fr>
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include <SDL.h>
+
+#include "clock.h"
+
+void
+clock_start(struct clock *clock)
+{
+	clock->ticks = SDL_GetTicks();
+}
+
+uint64_t
+clock_elapsed(const struct clock *clock)
+{
+	return SDL_GetTicks() - clock->ticks;
+}

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/clock.h	Mon Jan 06 21:27:45 2020 +0100
@@ -0,0 +1,55 @@
+/*
+ * clock.h -- track elapsed time
+ *
+ * Copyright (c) 2020 David Demelier <markand@malikania.fr>
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#ifndef MOLKO_CLOCK_H
+#define MOLKO_CLOCK_H
+
+/**
+ * \file clock.h
+ * \brief Track elapsed time.
+ */
+
+#include <stdint.h>
+
+/**
+ * \brief Clock structure.
+ */
+struct clock {
+	uint64_t ticks;         /* time point on initialization */
+};
+
+/**
+ * Start the clock and track elapsed time.
+ *
+ * \pre clock != NULL
+ * \param clock the clock
+ */
+void
+clock_start(struct clock *clock);
+
+/**
+ * Tell the measured time.
+ *
+ * \pre clock != NULL
+ * \param clock the clock
+ * \return the elapsed time in milliseconds
+ */
+uint64_t
+clock_elapsed(const struct clock *clock);
+
+#endif /* !MOLKO_CLOCK_H */