Alexander Belopolsky added the comment:
I've provide the direct quote from *C* standard ...
I understand that C standard uses the word "encoding", but it does so for a reason that is completely unrelated to the choice of epoch. "Encoding" is how the bytes in memory should be interpreted as "number of seconds" or some other notion of time. For, example "two's complement little-endian 32-bit signed int" is an example of valid time_t encoding, another example would be IEEE 754 big-endian 64-bit double. Note that these choices are valid for both C and POSIX standards.
If you google for your phrase "time in POSIX encoding", this issue is the only hit. This strongly suggests that your choice of words is not the most natural.