I've been doing this for years. The difference right now is 14 minutes (between computer clock and battery operated). When my computer clock is 1 minute faster than my landline phone, I have seen it jump back 1 - 2 minutes at time.

Doesn't take long to see the difference.⌛⏳⌛

This model is lying (leap seconds, yeah, right). None of the magnetosphere models are of much use.

https://hpiers.obspm.fr/eop-pc/index.php

https://hpiers.obspm.fr/eop-pc/index.php?index=C04&lang=en