I expected there would be constraints, but the chosen range is quite intriguing. The PostgreSQL spec says the 4-byte date type spans 4713 BC to 5,874,897 AD. It gives much more headroom for future dates—did they assume preserving data before 4713 BC is unlikely?

That range of dates seems to correspond to (1UL << 31) days so I suspect they're using only 31 bits so I wonder why they didn't make it signed and extend it to 5,884,322 BC.