The birthday date format is hardcoded with the MM/DD/YYYY format on two templates:

Profiles > ProfileModern

User Control Panel > membersProfileForm

This should be changed on future versions, since only the U.S. uses this format. Most countries use the DD/MM/YYYY format. It is not good practice to hardcode date formats.

Source: http://en.wikipedia.org/wiki/Date_format_by_country

It's no longer hard-coded in 4.0.

AWESOME