As c_schmitz says, you can't do this server-side but there are several client-side solutions. (It should be noted that these would only work if JavaScript is enabled in the respondent's browser and they don't actively hack the JS)
1) Validate the input before submitting the form. Add something like the following to the end of template.js. Replace "attribute_1" with your attribute field. (This snippet was developed for the default template and may need to be modified for others):
$(document).ready(function(){
$('.register-form form').submit(function(){
var attributeField = 'attribute_1';
var errorMessage = 'Please use date format mm-dd-yyyy';
var validFormat = /^(0[1-9]|1[012])-(0[1-9]|[12][0-9]|3[01])-(19|20)\d\d$/;
// Test date format
if(!validFormat.test($('input[name="register_'+attributeField+'"]').val())) {
alert(errorMessage);
return false;
}
});
});
2) Use a
jQuery date picker:
$(document).ready(function(){
var attributeField = 'attribute_1';
$('input[name="register_'+attributeField+'"]').datepicker({
dateFormat: 'mm-dd-yy'
}).attr('readonly', true);
});
3) Use an input mask -
digitalbush.com/projects/masked-input-plugin/ 
