If a user has *set* their timezone to be "UTC", rather than set it to be a timezone in the timezone database that is in +00:00, they have explicitly chosen to not be in a timezone. It is typically treated as a special agnostic timezone. It is therefore inappropriate to suggest changing their timezone when they change location/timezone.
(If a user is implicitly in "UTC" because they havent set any timezone in their prefs, that is a different case)
As I explained in T3025, I like to work in UTC, and this is the way my brain works with online collaboration in very large groups of people in widely dispersed geographical locations.
But when I am looking at my calendar app, I want to see local times, for whatever timezone I am in.
And, I dont have my laptop or tablet update its timezone, but I do on my smart phone, as that is my calendar and has roaming enabled, while the other devices do not. That means Phabricator keeps popping up this dialog whenever I switch between using my phone, tablet and laptop, because it believes I am flying back and forth between two countries ever half an hour or so. I am constantly having to ignore this conflict when I am travelling. It is *really* annoying.