diff --git a/date.c b/date.c
index 5d05ef61cf..45f3684cee 100644
--- a/date.c
+++ b/date.c
@@ -9,7 +9,7 @@
 /*
  * This is like mktime, but without normalization of tm_wday and tm_yday.
  */
-time_t tm_to_time_t(const struct tm *tm)
+static time_t tm_to_time_t(const struct tm *tm)
 {
 	static const int mdays[] = {
 	    0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334
diff --git a/git-compat-util.h b/git-compat-util.h
index 5c596875c2..85dea123a4 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -198,7 +198,6 @@ extern void warning(const char *err, ...) __attribute__((format (printf, 1, 2)))
 extern void set_die_routine(NORETURN_PTR void (*routine)(const char *err, va_list params));
 
 extern int prefixcmp(const char *str, const char *prefix);
-extern time_t tm_to_time_t(const struct tm *tm);
 
 static inline const char *skip_prefix(const char *str, const char *prefix)
 {