diff --git a/src/calc/Calc.js b/src/calc/Calc.js
index 4f71865..ba653ca 100644
--- a/src/calc/Calc.js
+++ b/src/calc/Calc.js
@@ -248,6 +248,8 @@ exports.relativeDate = function( data, value )
 {
     return _each( data, value, function( curdate, format )
     {
+        format = format || '';
+
         var type = format.substr( -1 ),
             tval = format.substr( 0, format.length - 1 ),
             ms   = 0;