Error with date validation

There is an error with the date validation in your clientside_validation.js file. Line 1084 (v7.x-1.33), you have to change
var month = parseInt(parts[param.monthpos], 10);
by
var month = parseInt(parts[param.monthpos], 10);
if (month>0) month--;

or something similar.

In Javascript, the month value is between 0 an 11, not between 1 and 12.