Probably more of a hat tip to this one:

https://www.ioccc.org/2000/natori/index.html

https://github.com/ioccc-src/winner/blob/master/2000/natori/...

It's interesting how the same 2551443 bitmask appears in the time calculation for both.

2551443 is the length of the synodic month in seconds. The calculation being done converts the current time in seconds into a 512-wide integer range representing the lunar phase by offsetting from the lunar phase during the unix epoch.