What your phone can and cannot do
iPhone can schedule a message through the Shortcuts app and Android has Schedule Send built into Messages. Both work, and both are designed around sending one message to one person.
Scheduling the same message to forty people means either creating a group thread, which exposes everyone to each other, or setting up forty separate scheduled messages. Neither is realistic.
That gap is the whole reason scheduled broadcasts exist. One message, one date, everyone.
The 12pm Eastern send time
Every scheduled message goes at 12pm Eastern on the date chosen. That is deliberate, and it is also the main constraint worth understanding.
It handles daylight saving correctly, so 12pm Eastern in June and in December are both actually noon in New York rather than drifting by an hour.
What it does not do is let you pick 6pm the night before, or 8am on the morning. If a specific hour is essential, this is not the right tool and it is better to know that now.
What time zones mean for your recipients
Midday Eastern is nine in the morning on the west coast and eleven in the middle of the country. For most announcements that range is fine.
Where it matters is anything tied to a local time, such as a store opening or an event starting. Writing the local time into the message itself solves it: doors at 6pm Pacific reads correctly wherever it lands.
Delivery is to US numbers only, so international recipients are not a timing problem so much as a reach problem. Wedding guest lists covers that gap where it comes up most often.
What is actually worth scheduling
The reminder is the highest value scheduled message there is. Two or three days before an event, when people have stopped thinking about it and have not yet remembered.
After that: holiday opening hours, a booking or payment deadline, the night-before message with an address, and anything you want going out on a day you will be busy.
Writing several at once while you have the time is the real benefit. A wedding, a reunion or a season of games can have most of its messages set up months ahead. Reunion planning is a good example, since it runs over months.
What it will not do
There is no recurring schedule. A weekly reminder means setting it up each week, since nothing repeats automatically.
There is no trigger of any kind, so nothing fires when someone books, pays or joins. Everything is set by you, by date.
And a scheduled message can be cancelled up to an hour before it sends. After that it is committed, which is worth remembering when plans change late.