diff --git a/MiniScript-cpp/src/DateTimeUtils.cpp b/MiniScript-cpp/src/DateTimeUtils.cpp
index b0da71d..8cf5984 100644
--- a/MiniScript-cpp/src/DateTimeUtils.cpp
+++ b/MiniScript-cpp/src/DateTimeUtils.cpp
@@ -13,7 +13,9 @@
 
 #if _WIN32 || _WIN64
 struct tm *localtime_r( const time_t *timer, struct tm *buf ) {
-	*buf = *_localtime64(timer);
+	struct tm *newtime = _localtime64(timer);
+	if (newtime == nullptr) return nullptr;
+	*buf = *newtime;
 	return buf;
 }
 #endif
@@ -31,7 +33,8 @@ static bool Match(const String s, size_t *posB, const String match) {
 
 String FormatDate(time_t t, String formatSpec) {
 	tm dateTime;
-	localtime_r(&t, &dateTime);
+	struct tm *newtime = localtime_r(&t, &dateTime);
+	if (newtime == nullptr) return "";  // arg t too large
 	
 	const int BUFSIZE = 128;
 	char buffer[BUFSIZE];
diff --git a/MiniScript-cpp/tests/testDateTimeDateStr.ms b/MiniScript-cpp/tests/testDateTimeDateStr.ms
new file mode 100644
index 0000000..b2ff00a
--- /dev/null
+++ b/MiniScript-cpp/tests/testDateTimeDateStr.ms
@@ -0,0 +1,8 @@
+import "qa"
+
+testDateTimeDateStr = function
+	qa.assertEqual _dateStr(0), "2000-01-01 00:00:00"
+	qa.assertEqual _dateStr(1e20), ""
+end function
+
+if refEquals(locals, globals) then testDateTimeDateStr