Countdowns: plan a short duration without confusing it with a deadline
A countdown answers “How much of my allocated time remains?”, not “What time is it?”. It suits a short exercise, presentation segment or focus period whose duration starts when you decide.
A duration or a fixed time?
Ten minutes is a duration; 15:00 is a time of day. If an activity starts late, its ten-minute duration can stay the same while its finish moves. An external appointment does not move because you started a timer later.
State the unit. A minute is 60 seconds and an hour is 60 minutes, so 1.5 hours means 1 hour 30 minutes, not 1 hour 5 minutes.
References: NIST — Units outside the SI: minute and hour
Pausing changes what the total means
Pausing and resuming a duration timer excludes the pause from the counted activity. That is useful for active work time, but it does not extend an external deadline.
For a sequence, add the planned activity periods and breaks once. Distinguish a scheduled break from an unplanned pause so the overall finishing time is not a surprise.
When a separate alarm is needed
A browser can delay a timer callback while its page is in the background or the device is asleep. Do not rely on a web-page timer alone for medication, a supervised examination or a time-critical appointment.
Keep ToolRoof open: reloading resets this timer, and sound is not guaranteed when device or browser settings prevent it. A countdown on screen is not a promise of an alert after the page closes.
References: MDN — Timers in inactive tabs
Worked example: three practice segments
Allocate 12 minutes to each of 3 segments, with a 3-minute break only between segments. Activity time is 36 minutes and the two breaks add 6, giving 42 minutes overall. There is no third break after the programme ends.
An extra 5-minute pause during a segment leaves the planned programme at 42 minutes but takes total wall-clock time to 47 minutes. That is the effect of the pause, not an arithmetic mistake.