Look, the only time travel metric for how far back or forward you've gone is one that is relative to the time/date/year you left.
BC/AD is not universal, and any attempt to set a universal measure would have to agree on a starting point.
This way, you could travel and know it's T minus 1200, or T plus 100. A fellow traveler might describe themselves in your time as T-50, for which you flip the sign and know for certain it's T+50 to you.
None of this Gregorian calendar nonsense. :-)