az_posix.c implements the az_platform_clock_msec by measuring CPU time, not wall time. This API isn't used today.
In ANSI C/POSIX, clock_t clock() is to approximate the CPU time spent in a process and the current naming creates confusion.

A few ways to fix this:

1. clock_gettime() clock_getres(). The problem is that POSIX doesn't guarantee availability of (all) timing functions. We'll need runtime code to discover enabled functionality.
2. Use the c time() epoch timer with 1 second accuracy.

Also, please consider the API breaking change to rename the API name to AZ_NODISCARD az_result az_platform_time_msec(int64_t* out_clock_msec) to further reduce confusion on the purpose of the API.