How to override date validation client side

I’m new in with vaadin framework I would like to extend the DateField and validate the date enterred by the user in “client side” server is already done

Example : if user write “today” is become “18.05.2015”

Thanks for all your help.

In server side it is quite easy to do this kinds of parsing manipulations.
Why you want to do it in browser?

DateField df = new DateField() {

      protected Date handleUnparsableDateString(final String dateString)
          throws Converter.ConversionException {
          return new Date();
        throw new Converter.ConversionException("UNABLE PARSE DATE");

Thank you Vesa for you help, because in my point of view when you put today in a date and is not evaluation right away is not good.

Server side is a double validation in case…