This page opens with the timer already set to 45 minutes. Press Start or hit the spacebar and it begins; when time is up the alarm rings and the disk blinks. You can always drag the disk to a different time.
Long blocks like 45 minutes tend to sag in the middle. Keeping the remaining time visible makes pacing easy — and the mini timer floats above other windows while you work.
45 minutes matches a school class period — great for one lecture video or a tutoring session.