A schedule belongs to the playlist, not to the screen. Assign that playlist to five screens and all five inherit its timing, each evaluating it against its own local clock.
There is no separate schedule object to create and attach. The Schedule tab on the playlist is the whole thing.
What a rule can restrict
Three things, and a rule can set any combination of them.
Dates. A start, an end, or both, plus individual dates to skip.
Time of day. All day, or specific hours.
Days of week. Individual days, or the presets Every day, Weekdays, and Weekend.
Anything you leave alone stays unrestricted. See Writing a rule.
With no rules at all
A playlist with no rules is Always active: eligible whenever it is assigned. That is what a new playlist starts as, and it is the right answer for the single playlist a screen shows all day.
Adding a rule narrows that. It never widens it.
What a schedule does not decide
Whether the playlist plays at all. A disabled playlist never plays whatever its schedule says.
Which screens it reaches. That is the Screens tab, a separate question.
What happens when two playlists are eligible at once. The screen plays both, in alphabetical order by title, unless one is a priority playlist. See When several playlists are eligible.
When it takes effect
Saving a schedule sends it to every assigned screen on its next check-in, and the screen stores it locally. From then on the screen evaluates the schedule itself, with no network involved, which is why scheduled content still changes on time through an outage.