diff --git a/ext/calendar/cal_unix.c b/ext/calendar/cal_unix.c
index b23215c8620..c57a82b4590 100644
--- a/ext/calendar/cal_unix.c
+++ b/ext/calendar/cal_unix.c
@@ -45,6 +45,10 @@ PHP_FUNCTION(unixtojd)
     t = time(NULL);
   }
 
+  if(t < 0) {
+	RETURN_FALSE;
+  }
+
   ta = php_localtime_r(&t, &tmbuf);
   jdate = GregorianToSdn(ta->tm_year+1900, ta->tm_mon+1,ta->tm_mday);