Typecasting to resolve warning: integer conversion resulted in a change of sign

platform/mbed_mktime.c

@@ -105,7 +105,7 @@ time_t _rtc_mktime(const struct tm* time) {
     }
 
     if (result > INT32_MAX) { 
-        return -1;
+        return (time_t) -1;
     }
 
     return result;

platform/mbed_rtc_time.cpp

@@ -53,7 +53,7 @@ time_t time(time_t *timer)
         }
     }
     
-    time_t t = -1;
+    time_t t = (time_t)-1;
     if (_rtc_read != NULL) {
         t = _rtc_read();
     }