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. :-)
@emanate This still relies on us all agreeing on a unit of measure!
@IrisKalmia It's true. One cycle around the sun, and smaller divisions, perhaps. There's never gonna be an absolute.
I've seen some sci-fi though offer "seconds" as a measure. Tying the second somehow into vibrating hydrogen or something. There's still going to be a metric, I just wanted to rant about AD/BC for a bit. :-P
@emanate Oh yeah! I've also seen some based around Planck Time, it's just a fun question to me