Here’s a screenshot from a form I was filling in on the TV Licensing website. I’d just entered the date as 9/6/2009 instead of 09/06/2009. Obviously it’s too tricky for them to work out how to pad 1-digit numbers with a leading zero.
This falls into the same category as sites that insist you enter your username in either lower or upper case. Or that you enter your credit card without spaces or hyphens. Or that you format your phone number in a particular way.
It’s all putting extra work on the user for something that could be done automatically server-side with just a few lines of code at most. Not good practice!