You can't specify a format for the open ended <specify> response option. However, you could check out the Advanced Features in MediaLab (see the User's Guide).
e.g., You could set up a complex skip for the multiple response item where answering the date type question sends you to a fill in the blank item that DOES restrict the formatting.
(Note that Multiple response items need to do skips via the Advanced Features. If you can do it with a Scale Response item instead, then it would be even simpler--as the scale item type allows you to set up simple skip patterns via the "skipto" option.)