Many time zones: You get to a new place and look up what time zone you’re in.
Well, sorta — but it’s no effort at all because my timekeeping device (phone) does this automatically.
For me, the time of day is internalized in a way that I think is hard to switch. Same as how I was raised with imperial units — even though I prefer (and use professionally) metric, the intuition can be a little harder to get. But to each their own of course :)
Diesel engine, Fischer-Tropsch, Homeopathy.