That good reason is that there is no general "formula" for leap seconds, unlike for leap years, they have to be looked up. So you can't do "offline" date calculations if they included leap seconds.

I think that UNIX time stamps are generally a very good approximation, and if you are comparing long enough time intervals for the error to get over one second, and/or that error to matter, you are doing something wrong anyway.

For exact time interval measurements that you have to get exactly right, don't use UNIX time stamps.

Because otherwise you would get annoying off-by-27 errors whenever you did time() % 60.

https://en.wikipedia.org/wiki/Leap_second#Binary_representat...