Not sure weather related to theme. But using this wonderful theme.
Try:
1) create date field in siteadministration/Users/Accounts/user profile field (in my case: required: no; locked: no; display: yes; not visible) in additional category.
2) go to you user and put (i.e. todays) date; save
3) Try to disable the entered date (= clicking checkbox off) -> Jan 1 2013 (start year) will remain; checkbox remains enabled.
Greetings Ivo