I don't like having to enter a date using a specific format like 'mm/dd/yyyy' (as a string, the slashes or dots of course being mandatory as well.) There are libraries whose entire purpose for being is turning datelike strings into timestamps, and anything that doesn't parse with strtotime (or what have you) can just return an error.

There's really no reason i can think of why you can't just enter any number of natural language strings other than laziness on the part of the programmers.

Somewhat related: I hate entering a date by using two (credit card expiration) or three (date of birth, etc) different drop-downs.