diff --git a/firmware/common/timefuncs.c b/firmware/common/timefuncs.c
index d073ec819c..9865b0fe17 100644
--- a/firmware/common/timefuncs.c
+++ b/firmware/common/timefuncs.c
@@ -131,6 +131,7 @@ int set_time(const struct tm *tm)
 #endif
 }
 
+#if CONFIG_RTC
 /* mktime() code taken from lynx-2.8.5 source, written
  by Philippe De Muyter <phdm@macqel.be> */
 time_t mktime(struct tm *t)
@@ -165,3 +166,4 @@ time_t mktime(struct tm *t)
     result += t->tm_sec;
     return(result);
 }
+#endif
diff --git a/firmware/include/timefuncs.h b/firmware/include/timefuncs.h
index d2f42eb9ff..e266ff9acb 100644
--- a/firmware/include/timefuncs.h
+++ b/firmware/include/timefuncs.h
@@ -27,6 +27,8 @@
 struct tm *get_time(void);
 int set_time(const struct tm *tm);
 bool valid_time(const struct tm *tm);
+#if CONFIG_RTC
 time_t mktime(struct tm *t);
+#endif
 
 #endif /* _TIMEFUNCS_H_ */