Yes, March 31 is J.S. Bach's birthday, because during his pre-teenage years, the Julian calendar was changed to the current Gregrorian calendar, with a change of 10 days, so March 21 + 10 = March 31.
So, it's JS Bach's 334th birthday!!
And, it so happens to be my birthday as well, so I - of course!! -:) -- totally agree to celebrate both today!
Here is short version of the now world famous "Happy Birthday to You" song, with a bit of B-A-C-H tonality added to it.