What's the best approach to get Date/Time input from the user?

Question

This is a wheel that\'s been re-invented again and again over the years.

The Problem: The user needs to input a Date/Time

Basic considerations

Answer 1

My preference is for a text input with an elipsis button next to it:

Enter a date [        ] [...]

The elipsis would pop up a calendar to populate the text input, but the user can type in a date if they want. Validation should be done when the "OK" button for the form is pressed - trying to do date validation on a character by character basis is doomed, in my experience.

The validation should be sophisticated and allow expressions like

"today"
"Tomorrow"
"23 Jan"

etc.

Edit: In reply to some comments, one could do validation when the text edit loses focus (though I hate that kind of thing) in which case the edit content could change from "23 Jan" to "23-01-2009" to indicate that the exprssion was understood.