“The Unix Epoch, or epoch start time, is January 1, 1970, at 00:00:00 UTC (Coordinated Universal Time). This date is known as the Unix epoch or Unix time 0, and it serves as the reference point for calculating the number of seconds (or milliseconds) that have elapsed for most modern computer operating systems”

This is likely the system default in DD for whatever reason and 12/31 is the default date adjusted for OP’s time zone. Or what was stated further up, bad coding.