What could a timestamp like 637946761073844996 mean?

I'm struggling with obscure XML data which contain pieces like this:

<CreationDate>637774857317419771</CreationDate>
<ModificationDate>637946761073844996</ModificationDate>

Now I'm wondering what that could mean. I twiddled around a bit (using Python as calculator) and found this:

>>> (637946761073844996 - 637774857317419771) / 3600 / 24 / 1e9
1.989626810477141

Two days between creation and modification date is plausible, the data in question was generated just a few months ago. So maybe the timestamp is nanoseconds since some time ago. Let's try:

 >>> 637946761073844996 / 3600 / 24 / 1e9 / 365.25
 20.215312985583346

So maybe, just maybe, the thing is counting nanoseconds since about 2002. I could do some research, in fact I have access to the black box that produces this data, so I could create something at a known time, then modify it at some other time, evaluate the results and use that as the basis.

But then I maybe totally wrong, and maybe this is a "standard" timestamp of sorts that somebody recognizes. Like one immediately sees that 1662561732 is a UNIX timestamp (if you know it's a timestamp, and that it is fairly recent).