A calendar is built in two steps: create the calendar itself, then add the open periods that make up its week.
1. Create the calendar
Give it a name and a timezone. Name it for what it represents — "UK Business Hours", "24x7 Critical" — rather than for the team currently using it.
2. Add its hours
Add an entry for each open period, giving the day of the week and the start and end times. A standard week is five entries — Monday to Thursday 09:00 to 17:30, Friday 09:00 to 17:00, for example.
Saturday and Sunday have no entries, so the calendar is closed then. A day with no entries is closed — there is nothing to switch off.
Split days
Add two entries for the same day to model a break — 09:00 to 13:00 and 14:00 to 17:30, for a calendar that genuinely closes at lunchtime.
A 24×7 calendar
Add an entry for each of the seven days running 00:00 to 23:59.
Timezone
Hours are read in the calendar's own timezone. An organisation spanning several regions should have a calendar per region rather than one calendar and a lot of mental arithmetic.