Don't try to work with negative timestamps

# It returns the same day as for 0, which is wrong

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);