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

There may be at least 8 different answers to "how many seconds have elapsed since 1972-01-01 until now" depending on legal jurisdictions: http://www.ucolick.org/~sla/leapsecs/epochtime.html

[ed: added second link]

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?

There's a standard for what this is trying to do: TAI, or TAI with an offset of 19 seconds, like GPS.

Rolling your own is never a good idea.

Yes, this equates to TAI minus 35 seconds. GPS did exactly the same thing (instantiated at a TAI offset equal to UTC), perhaps in a vain hope that the astronomers would give up their death-grip on time. Not sure the difference between 35 and 19 amounts to "rolling your own."

(At least future decisions are out of the hands of the astronomers and into the hands of the timekeepers, so maybe these responsible authorities will just refuse to declare further leap seconds regardless of heavenly tantrums.)

The comment just describes a trivial notation for representing 128-bit TAI as an unsigned integer. The closest comparable "standard" that I know of is DJB's TAI64 [1]. (Everything released by DJB should be regarded as a standard.) But TAI64 is (a) defined as a bytestream, not a uint128_t; and (b) has a hopelessly complicated fractional subsecond system.

[1] http://cr.yp.to/proto/tai64.txt.

This scheme is different due to the way it handles times between the start of leap seconds and 2012.

And, as an astronomer turned technologist, I think your comments are completely inappropriate for civilized discussion. If you want verbal mud-wrestling, take it elsewhere.