The hours two or more places are open at the same time — often none.
An overlap window is the span during which every place on a list is inside its working hours simultaneously. It is the quantity anyone scheduling across regions actually needs, and it is not obvious from a list of offsets.
It shrinks fast. Two European offices overlap most of the day; add New York and it falls to a few hours; add Sydney and for many combinations it reaches zero, meaning no hour of any day exists in which all three are at work.
Zero is a real answer and worth stating plainly rather than presenting a least-bad hour as though it were a solution. The honest output is that the meeting cannot be held inside working hours and somebody has to be inconvenienced deliberately.
Offsets tell you what time it is somewhere. The overlap window is what you get after crossing them with each place's working hours and its rest days.
See also utc offset · time zone · local time · rest day · working day
The Operating Calendar computes this across any number of places, using each one's own rest days rather than an assumed Monday-to-Friday week. See what we hold.