You won't be able to use ext4 though.
Edit: Unless you already...
Also, wouldn't the kernel keep the original time? Time conversion could be performed in userland, with big integers.
Age of the universe in seconds ~= 4.3 x 10^17 [1]
So assuming you're using an unsigned int to represent the number of seconds from the big bang, you'll still have ~ 10^21 seconds left on the clock (31.7 trillion years).
Unless you're working with a timeline that extends out to the heat death of the universe (10^100 -> 10^(10^56) years), 128 bits should be enough.
[0] https://www.wolframalpha.com/input/?i=2%5E128
[1] https://www.wolframalpha.com/input/?i=age+of+the+universe+in...
If the heat death of the universe is 10^100 years, that's 6*10^150 plank times [1]. log_2(that) ~ 500 bits. So you'd probably go with 512 bits for a maximally-precise time type that will never need to be updated for either size or precision.
[1]: http://www.wolframalpha.com/input/?i=convert+10%5E100+years%...
- Quantization of time is not a part any widely accepted physical theory, and the evidence we have so far is consistent with time being continuous. Timekeeping with Planck time units would be "nice" (well, "overkill" and "impossible" are two good words to come to mind as well), but there's no reason in theory that you can't go finer.
- When your time is this fine, you also need to talk in very specific ways about reference frames. Time passes at different rates at different altitudes, and relative velocities also screw with you. Where's your reference clock? In some inter-galactic space, stationary relative to the CMB?
With that context I (arbitrarily) chose plank time to be the smallest possibly-ever usable (maybe not practical) time scale for timekeeping.
That's a good point about location. If we're going for extremes, we should put it in one of the especially large spaces between galaxy clusters: supervoids[0]! If there were a clock made at plank time frequency, I wonder how much of an effect structures would have on it at interstellar, intergalactic, and void space distances. At this accuracy (and these distances) I imagine the clock's own gravity would be a major source of time dilation.
Perhaps the best setup would be an extremely small (think the size of a virus) clock with a small support station that provides energy to it and reads ticks back, kept a few hundred thousand light-years away to avoid gravitational influence.
Another thing. 10^100 years is a long time. So long that if the time from the big bang to the heat death of the universe were one second long, the universe isn't even 1 plank time old yet. 10^100 years is so long it starts making sense to talk about the half-lives of elements that are considered perfectly stable. To make any clock that is designed to operate continuously for that long would be a crazy undertaking even for an interstellar civilization.
http://wolframalpha.com/input/?i=age+of+universe+%3C+%282^12...
You have: 2^128 seconds
You want: yottayears
* 10783128
But if you want to measure in seconds, 64 bits are enough already.