In the enhanced version of the episode it mentions the Black Rock left on March 22nd.
I'm always curious to see why Lost choses the dates it does.

This date seems to be more of an inside joke.Carlton Cuse and Tania Raymonde (Alex) were both born on this date.

Every date Lost has used so far has had some significance.I thought it was interesting the significance of this one was the birthdays.

Macainickle