There is no "0AD", and if you don't have leap seconds then the number of seconds per repeat of the Gregorian calendar is not fixed—just have a look at the graph at http://www.ucolick.org/~sla/leapsecs/deltat.html
TLDR: if you define a bunch of complexity into your definition of time, you'll have a very complex model of time.
Eg, if you define the Gregorian calendar as a function of the position of the planets, rather than a function from atomic-clock seconds to a string, then the Gregorian calendar is not fixed. I suppose Pope Gregory designed it as a function of astronomy, so who are we to roll our own?
Eg, if you define the Gregorian calendar as a function of the position of the planets, rather than a function from atomic-clock seconds to a string, then the Gregorian calendar is not fixed. I suppose Pope Gregory designed it as a function of astronomy, so who are we to roll our own?