What are the possible values of _POSIX_TIMERS?

The posixoptions manpage shows that the following macros can be used to determine the degree of a host's support for the posix timers API:

• _POSIX_TIMERS
• _POSIX_CPUTIME
• _POSIX_THREAD_CPUTIME
• _POSIX_CLOCK_SELECTION
• _POSIX_MONOTONIC_CLOCK

On my dev system (Ubuntu-based Linux distro, using gcc 7.5.0), _POSIX_TIMERS evaluates to 200809, _POSIX_CPUTIME to 0, and _POSIX_THREAD_CPUTIME to 0.

I have been searching for more information on these macros, but can't seem to find the needed information. I would like to know where they are defined (in a header file? or "magically" by the compiler?), and what the possible values of _POSIX_TIMERS are.

However, I did run across some C code which compared _POSIX_TIMERS to another constant starting with 2012. So obviously various different values must be possible.

The POSIX definition (2018 edition) says that:

_POSIX_TIMERS

The implementation supports timers. This symbol shall always be set to the value 200809L.

It's defined in the <unistd.h> header.