That's clever. I might steal that at some point. Or combine into a millisecond_of_month on a 32 bit processor.

For people that don't play with bits often: Unsigned 16-bit integers have their max as 65,536. Storing data often wastes most of the bits, since a field that goes from 0-1000 doesn't fit in an 8-bit integer, and C doesn't have native integers between 8 and 16 bits long. (You can mask out relevant fields, but that's annoying.)

So there are 1000 * 60 = 60,000 milliseconds in a minute, which is an impressively good use of the space available. There are also a max of 60 * 24 * 31 = 44,640 minutes in a month, which is pretty good usage of the second value as well. The final 16 bits are probably 4 bits for month and 12 bits for the year, going up to 4096. Alternatively it could be using it as something like month_of_common_era to get it out to year 65536/12 = 5461.

It also cooperates with leap seconds, by counting faithfully up to 61,000 on such a minute.

IIRC, month and year was a 4-bit and 12-bit bitfield just as you describe.