[phaker]

TAI is the 'UTC without leap seconds'.

UT1 is an evolution of solar time, and wobbles when Earth does because it's using the Earth and Sun (and other bodies) as a clock.

UTC is TAI with leap seconds added when UT1 drifts too far away.

[jws]

I wrote an OS that used TAI as it’s time. All in all I was pleased with the outcome. UTC conversion happened at parsing and formatting, but those are relatively uncommon compared to comparison and interval addition so it works well performance wise.

[LeoPanthera]

"UT1R" is a smoothed version of UT1 with the wobble removed.

[aidenn0]

Given that UTC is always within 0.9s of UT1, either TAI or UT1 can be considered "UTC without leap seconds" since the leap seconds in UTC are an attempt to reconcile the two.