Something which bugs me about UNIX time is that, contradictory to its colloquial name (epoch time), UNIX time does not increase linearly and does funny stuff on leap seconds.
Apparently newer Linux kernels support CLOCK_MONOTONIC_RAW which is not affected by NTP, but even that may not increase linearly: it’s not updated when the system is in standby.
Then there is also CLOCK_BOOTTIME which is monotonic and accounts for time spent in standby.
Neither of these seem to be POSIX standardised, though.
https://linux.die.net/man/2/clock_gettime
This clock is not affected by discontinuous jumps in the system time (e.g., if the system administrator manually changes the clock), but is affected by the incremental adjustments performed by adjtime(3) and NTP.
If the adjustment in delta is positive, then the system clock is speeded up by some small percentage (i.e., by adding a small amount of time to the clock value in each second) until the adjustment has been completed. If the adjustment in delta is negative, then the clock is slowed down in a similar fashion.Not all epoch time counters are UNIX time though.
The usual case, when referring to an epoch time counter being used, is a uniform, increasing count of elapsed time in a standard fixed length unit (seconds, cycles, orbits, etc.).