You add a whole number of hours and for some a half
Or three quarters in a few cases.
And of course there are cases where countries spanning as many as 5 “ideal” time zones (dividing the globe into 24 equal slices) actually use a single time zone.
And then when someone tells you the meeting is at 10:00 am, you have to figure out if they mean your time zone or theirs, and if they mean theirs, you then have to convert that to yours. Oh, but your conversion was wrong because one of you went into or out of daylight saving time between the day when you did the conversion and when the meeting took place.