It is as I thought on the Pi, which, is running a 32-bit OS (if what I read is correct).
This is the calculation method currently, from my Pi 3, using 30 days:
[10-11-2017 @ 21:11:47.652 MDT] >> 1507777907651 -1702967296
[10-11-2017 @ 21:11:47.687 MDT] >> 1507777907686 -1702967296
The last number of the number of milliseconds in a day times the number of days configured, as one can see, it rolls over. How so, you ask? A 32-bit integer, unsigned max is 4,294,967,295. Subtract 2,592,000,000 from 4,294,967,295 and you get 1,702,967,295. The above number, off by one due to roll over and the storage of the negative bit.
So, if we calculate by seconds instead of milliseconds:
[10-11-2017 @ 21:15:00.229 MDT] >> 1507778100 2592000
We have a much better number when using 30 days and the file isn’t automatically removed:
illusion% ls dbbackup/
Fixed in 1711. The fix should allow you to go up to over 24,000 days. I assume no one needs that many backups.