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Open the Control Center (click the ClassTimer toolbar button, then Open Control Center, or press Ctrl Shift 7). Type a number of minutes and press Enter. Decimals work: 2.5 is 2:30. Add s for seconds, as in 45s.

Click Open floating timer (or press F) and drag the window onto the projector screen. Resize it from its edges; the digits grow to fill it. Chrome usually remembers where you put it.

You can also open it from the toolbar button without the Control Center: choose Open floating timer, then press Enter in the small window that appears. That window minimizes itself.

Chrome closes a floating window when the page that opened it closes: the Control Center tab, or the small launcher window. Keep that open (pin the tab, or leave the launcher minimized). The countdown itself keeps running; click Reopen floating timer to bring the window back.

Chrome draws that bar on every floating window so a page can't disguise itself. Extensions and websites can't remove or restyle it.

No. Chrome's floating windows are always solid. The background opacity setting darkens the background instead.

It stays above your other windows, including Chrome in full screen. Whether it stays above another program's full-screen mode, such as a PowerPoint slideshow, depends on that program and on Windows, so try it once in your room. To hide the time for a moment without stopping it, press H or Ctrl Shift H.

5 Enter	Start a 5:00 timer (any number of minutes)
Space	Pause or resume
A	Add 30 seconds
H	Hide or show the time
F	Open or close the floating timer (Control Center)
Ctrl Shift 9	Add 30 seconds from anywhere in Chrome
Ctrl Shift 8	Pause or resume from anywhere in Chrome
Ctrl Shift H	Hide or show the time from anywhere in Chrome

Single keys work in the window you last clicked. Change the Ctrl+Shift shortcuts at chrome://extensions/shortcuts.

In the Control Center, open Schedules and choose New school. Enter your periods' start and end times, tick the periods you teach under My periods, choose which schedule each weekday uses, and click Save changes. ClassTimer won't save a schedule with overlapping periods or impossible times, and tells you what to fix.

Add a special day in Schedules for that date and pick the schedule, or No school. For a one-off change, use the Today menu on the Timer page; it switches back automatically tomorrow.

Yes. Turn on Sync in Settings. Your schools, schedules and settings are kept in Chrome sync, in your own Google account, and appear on other computers where you're signed in to Chrome. You can also export a backup file in Settings and restore it on another computer.