If Bitcoin time value is 4 bytes, is that Unix time from 1970?
It is a Unix timestamp, but let me correct a minor detail:

The Y2.038 problem applies to a signed 32 bit number. The Bitcoin timestamp is unsigned, so it can represent the date for twice as long. Therefore, it will become a problem in 136.1 years, or in the year 2106.

Somewhen before then, there will need to be a hard fork.