What counts as a reasonable hour to ask of someone

Every scheduling tool will happily tell you that 07:00 and 20:00 are both inside a fourteen-hour window and therefore both available. Whether either is reasonable is a different question, and it is the one that decides whether people show up alert or resentful. It is worth having an explicit answer rather than an implicit one, because the implicit answer is almost always "whatever suits the person who booked it".

The core hours most people actually share

In practice the block that causes least friction across cultures is roughly 09:00 to 17:00 local, minus the middle of the day. That is not a rule handed down from anywhere. It is simply where school runs, commutes and lunch tend not to be, in the largest number of places at once. Treat it as the default and anything outside it as a request that needs a reason.

Where the default does not hold

The working week differs. Much of the Gulf works Sunday to Thursday. Friday is a normal working day in some countries and a short day or a day off in others. A recurring Friday slot can quietly exclude a whole region.
Lunch is not universal. A midday call is unremarkable in some countries and mildly rude in others, where the middle of the day is a genuine break rather than a sandwich at the desk.
The day starts and ends later in some places. In parts of southern Europe and Latin America a 09:00 start is early and an 18:30 finish is ordinary. Shifting a call an hour later can turn an awkward slot into a comfortable one.
Public holidays are local. They rarely appear on a foreign colleague's calendar, and school holidays differ even within one country.

Hours worth protecting outright

A short list of hard limits does more for scheduling than any amount of goodwill. The ones that hold up:

Nothing before 07:00 or after 21:00 local for any participant.
No recurring meeting that lands on someone's evening every single week.
No meeting that crosses local midnight for anyone, because it moves the event to a different date in their calendar and makes every later reference to it ambiguous.
No scheduling on someone's public holiday unless they have said it is fine, individually and recently.

Asking rather than assuming

The most useful thing you can do when a team spans continents is to ask each person for their own window rather than inferring it from their time zone. People differ inside a zone more than the zone differs from its neighbour. Some prefer an early start to protect the evening, others the reverse. Once those windows are written down somewhere shared, the search for a slot becomes mechanical and stops being a negotiation.

The meeting planner shows the overlap for the cities you pick, colour-coded by how comfortable each hour is on each side, which makes the trade-off visible before you send the invitation rather than after. Choosing to inconvenience someone is sometimes unavoidable. Doing it without noticing is what people mind.