> enough bits to go backward from the epoch at t=0 to the big bang at t=negative whatever.
For reference, the universe is about 13.787 billion years old [0]. That's about 13.787 * 10^9 * 365 * 24 * 3600 = 4.348 * 10^17 seconds, which (I think?) is a 59-bit number [1]. 10ths of a second will require 62 bits, which is right about at the edge of what a 64-bit signed integer will allow.
If you want milliseconds, you'll need at least 69 bits. For nanoseconds, you'll need at least 89 bits.
So you'll either need an integer type that's wider than what's natively supported in most hardware (thus potentially sacrificing performance), or you'll have to sacrifice precision.
So, the problem is a 64 bit int only gives you ~500 years around the epoch at nanosecond resolution. That's sufficient for timestamping, but not great for a general purpose time library where even ordinary civil calculations might exceed that range, particularly in intermediate arithmetic.
Since larger than 64 bit ints are a disaster for portability, the reasonable solution is to go with a 64 bit signed seconds, 32 bit nano offset field. A lot of language std libs have adopted something along these lines.
DJB was advocating for everything to be in a format like this, referenced to TAI (UTC without leap seconds, basically). Sadly that didn't get any traction.
I remember in my University years i was astounded to find out that none of the programming languages we were taught could represent the Jurrasic period, or the day the earth was formed, as a date.
I was horrified even more when i learned that future leap seconds are undefined, and we literally can't tell what is the time on the clock lik3w a million seconds from now.
For reference, the universe is about 13.787 billion years old [0]. That's about 13.787 * 10^9 * 365 * 24 * 3600 = 4.348 * 10^17 seconds, which (I think?) is a 59-bit number [1]. 10ths of a second will require 62 bits, which is right about at the edge of what a 64-bit signed integer will allow.
If you want milliseconds, you'll need at least 69 bits. For nanoseconds, you'll need at least 89 bits.
So you'll either need an integer type that's wider than what's natively supported in most hardware (thus potentially sacrificing performance), or you'll have to sacrifice precision.
[0]: https://en.wikipedia.org/wiki/Age_of_the_universe
[1]: https://www.wolframalpha.com/input/?i=13.787+*+10%5E9+*+365+...