LibGetTime():
- Two variables were used for the epoch, where only one should have been [*].
- Also harmonize variable name to match the one used in LibSetTime.
LibSetTime():
- Address possible underflows if time is set to start of epoch.
- Ensure that time being read does actually match time that was manually
set (plus the time elapsed since), by subtracting number of seconds
since reset.
[*] This fixes a build breakage, since one of these variables was set but
never used, triggering a compiler diagnostic at some optimization levels.