How to override date validation client side

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() {

      @Override
      protected Date handleUnparsableDateString(final String dateString)
          throws Converter.ConversionException {
        if("today".equals(dateString)){
          return new Date();
        }
        throw new Converter.ConversionException("UNABLE PARSE DATE");
      }
    };