This is a question more out of curiosity than need to solve a problem, but I'm not sure where else to find the answer, so I turn to this group of expert uses.
I know that FileMaker Pro stores dates as pure numbers, beginning at some point in the past, which I'll call D1 for "Day 1". I'm also pretty sure that D1 is not January 1 of the year 1 CE. But I've heard several different dates mentioned as to what D1 really is, and it's apparently different from one context to another. (Your operating system or Excel, for instance, may use a different D1 than FileMaker does.) What does FileMaker use as its D1?
And, if we want to refer to dates earlier than that (say for a database of historical records or genealogies), does FileMaker compute them (and then presumably store them) as negative numbers?