diff --git a/platform/mbed_mktime.c b/platform/mbed_mktime.c
index 49b4a4a610..6954e723f4 100644
--- a/platform/mbed_mktime.c
+++ b/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;
diff --git a/platform/mbed_rtc_time.cpp b/platform/mbed_rtc_time.cpp
index 6f26fb27a8..7a0ef48834 100644
--- a/platform/mbed_rtc_time.cpp
+++ b/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();
     }