I have some FileMaker timestamp which I don't know how to handle. (I discovered it by trial...)

Does someone know an algorithm to convert FM (File Maker) timestamp into DateTime?

I have read about the format on this page. Which includes a "FM dec Timestamp" button which makes the desired conversion, but gives no reference on how it does so!

Also, my timestamps differs in format from the one required in the site, mine has a size of 18 digits, whearas the site only allows 11.

Inserting 634890864000000000 and removing the trailing zeroes (to leave 11 digits), I got this date:

Wednesday, 2012-11-21 10:20:00