diff --git a/include/datetime.php b/include/datetime.php
index 3b1491e4d8..f6bf1041e1 100644
--- a/include/datetime.php
+++ b/include/datetime.php
@@ -87,7 +87,7 @@ function datetime_convert($from = 'UTC', $to = 'UTC', $s = 'now', $fmt = "Y-m-d
 		$from = 'UTC';
 	if($to === '')
 		$to = 'UTC';
-	if($s === '')
+	if($s === '' || (! is_string($s)))
 		$s = 'now';
 
 	// Slight hackish adjustment so that 'zero' datetime actually returns what is intended