Set one or more alarms — they ring while this tab stays open. Use Full screen for a bedside clock.
Tap Set alarm, pick a time and (optionally) a label, then choose one time or a repeat: daily, weekly on chosen days, monthly on a date, or monthly on a weekday like the 3rd Tuesday. Tap Edit on any alarm to change it. Full screen gives a big landscape bedside clock with Set, Edit and Stop on the face.
The alarm rings only while this page is open, so leave the tab open if you're relying on it. Counting down to a date instead? Make a countdown, or set a timer.