30 Minute Timer
Set for 30 minutes. Press start — it rings when the time is up.
Free from Ticka — the reminder app that rings like an alarm, even on silent. Close this tab and the timer stops. On your phone, Ticka rings even on silent. App Store → Google Play → App Store → Google Play →
This timer runs while the tab is open. If you need it to go off when your phone is in your pocket — that part needs the app.
Half an hour as a planning unit
Thirty minutes is the default unit of the calendar, not of the work. Meeting invites open at thirty; scheduling tools slice the day into halves of an hour; a thing that needs eight minutes gets thirty because thirty is what the grid offers.
Used deliberately, it is a good size: long enough to make real progress on something, short enough to fit between other commitments and to hold in your head from beginning to end. Used by default, it quietly inflates every task to the size of its container.
The useful move is to set the timer before you decide the task is a half-hour job. If it rings while you are still going, the work was bigger than the grid — worth knowing. If you finish in twelve minutes, you have just recovered eighteen the calendar would have eaten.
If the half hour has to reach you away from this screen, set a reminder on your phone for thirty minutes from now.