Date Validation

I am trying to validate a date field from within my model file but
running into some issues. If I put invalid data in a textbox on a form,
by the time it gets to my validate method, something has already emptied
the data out because it does not meet the data type specs (date field).
I could
validate in the controller before I call a save, but then I can’t add to
the errors collection and this doesn’t seem like the best practice
either. Does anyone have any suggestions? I am not using drop down lists
for the date fields, just a plain textbox which is needed for the
business requiremnet. Thanks for any advice!