diff --git a/include/util/time.h b/include/util/time.h
index d4b5cca7..6b35ef5f 100644
--- a/include/util/time.h
+++ b/include/util/time.h
@@ -13,4 +13,10 @@ uint32_t get_current_time_msec(void);
  */
 int64_t timespec_to_msec(const struct timespec *a);
 
+/**
+ * Subtracts timespec `b` from timespec `a`, and stores the difference in `r`.
+ */
+void timespec_sub(struct timespec *r, const struct timespec *a,
+		const struct timespec *b);
+
 #endif
diff --git a/util/log.c b/util/log.c
index 17655d30..cbec3e86 100644
--- a/util/log.c
+++ b/util/log.c
@@ -8,6 +8,7 @@
 #include <unistd.h>
 #include <wayland-server-core.h>
 #include <wlr/util/log.h>
+#include "util/time.h"
 
 static bool colored = true;
 static enum wlr_log_importance log_importance = WLR_ERROR;
@@ -27,17 +28,6 @@ static const char *verbosity_headers[] = {
 	[WLR_DEBUG] = "[DEBUG]",
 };
 
-static void timespec_sub(struct timespec *r, const struct timespec *a,
-		const struct timespec *b) {
-	const long NSEC_PER_SEC = 1000000000;
-	r->tv_sec = a->tv_sec - b->tv_sec;
-	r->tv_nsec = a->tv_nsec - b->tv_nsec;
-	if (r->tv_nsec < 0) {
-		r->tv_sec--;
-		r->tv_nsec += NSEC_PER_SEC;
-	}
-}
-
 static void init_start_time(void) {
 	if (start_time.tv_sec >= 0) {
 		return;
diff --git a/util/time.c b/util/time.c
index a47fc613..aed577de 100644
--- a/util/time.c
+++ b/util/time.c
@@ -4,6 +4,8 @@
 
 #include "util/time.h"
 
+const long NSEC_PER_SEC = 1000000000;
+
 int64_t timespec_to_msec(const struct timespec *a) {
 	return (int64_t)a->tv_sec * 1000 + a->tv_nsec / 1000000;
 }
@@ -13,3 +15,13 @@ uint32_t get_current_time_msec(void) {
 	clock_gettime(CLOCK_MONOTONIC, &now);
 	return timespec_to_msec(&now);
 }
+
+void timespec_sub(struct timespec *r, const struct timespec *a,
+		const struct timespec *b) {
+	r->tv_sec = a->tv_sec - b->tv_sec;
+	r->tv_nsec = a->tv_nsec - b->tv_nsec;
+	if (r->tv_nsec < 0) {
+		r->tv_sec--;
+		r->tv_nsec += NSEC_PER_SEC;
+	}
+}