There's also coconutBattery as an option, a small utility app that provides basic information about a Mac regardless of whether it's got a battery installed or not. The actual date is provided by coconutBattery, compared to the week-of-manufacture by the chipmunk.nl portal; I've compared the coconutBattery-provided manufacture date to the date provided by Apple during Genius Bar visits with no reason to question either as they generally match.
So, this 9-year-old thread comes back to coconut utility apps…