The U
and G
are not WordPress specific. get_post_time()
is using the same datetime formats as the default PHP date
method.
Here is the documentation for all of the formats:
https://www.php.net/manual/en/function.date.php
Specific to the question:
U
is Seconds since the Unix Epoch (January 1 1970 00:00:00 GMT)
G
is 24-hour format of an hour without leading zeros. 0 through 24