Is there any standard way tkinter apps allow the user to choose a date?
No, but you can get it from the user as a datetime element from a formatted string..
Example:
import datetime
userdatestring = '2013-05-10'
thedate = datetime.datetime.strptime(userdatestring, '%Y-%m-%d')
Check out http://docs.python.org/2/library/datetime.html#strftime-strptime-behavior. It's handy, although not the most user friendly way of getting a date.