Set one or more alarms — they ring while this tab stays open. Use Full screen for a bedside clock.
Tap Set, 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; Compact and Show on top give a small floating clock.
The alarm rings only while this page (or its floating window) is open — here's exactly how it works. Counting down to a date instead? Make a countdown, or set a timer.