Date format validation problem

[geinbest]

Although I've set the date format to DD/MM/YYYY for an obligated date text field, I still get the error message saying that the value is not valid when picking a date using the datepicker or manually entering the date in DD/MM/YYYY format.

For example: The date 16/05/2009 is not valid, I need to change it to 05/16/2009 before I can send the form.

I've also checked the Joomla default date format in the file languages\nl-NL\nl-NL.ini. Everything in this file is based on the date/month/year format.

Any ideas how to solve this problem?
Thx in advance!