If you set your clock to UT1 then it will be “wrong” by up to 0.9 seconds fast or slow compared to UTC, and the length of your seconds will be “wrong” by up to

If you set your clock to UT1 then it will be “wrong” by up to 0.9 seconds fast or slow compared to UTC, and the length of your seconds will be “wrong” by up to about 1e-8 compared to SI seconds. (Before its recent speed-up, the earth took about 24x60x60+0.001 seconds to rotate.) One of the key reasons for moving away from UT1 or UT2 as the standard broadcast timescales up to 1972 was that the time broadcasts were (and are still) also frequency broadcasts, so when they adjusted the length of the second to match the earth, they also had to retune their high-power transmitter so that the advertised number of cycles per second was correct. And when this retuning happened they also had to promulgate the new adjustment factor to all their users who needed actual SI or ephemeris time seconds instead of rubber UT1 seconds.