Date Field?

[ZiNgA BuRgA]

It's probably a little convoluted, though you could make a day+month+year field and then edit templates to display them on one line.  Make each of them selectboxes and set Hide Input Field = Yes.  Then make a copy of the newthread and editpost_first templates, with custom forum prefix and then edit this copy sticking in HTML for the field.  You can use {$tfinput['key']} to denote the HTML of the input field itself.

Sorry for the quick explanation - hopefully it makes sense.