Countdown Timer End Time Calculator

How it works

end (seconds after midnight) = start time in seconds + rounds × timer length + (rounds − 1) × break · end clock = end mod 86,400 · day offset = ⌊end ÷ 86,400⌋ · start time = finish − timer length (wrapping back past midnight) · timer length = finish − start (+86,400 if the finish is earlier on the clock)

Enter how long the timer runs and when it starts, and the calculator gives the exact clock time it reaches zero, to the second, with a flag and the end date when it runs past midnight. It turns the start time into seconds after midnight, adds the timer length (hours, minutes and seconds), then wraps the result at 86,400 seconds, one full day, to get the clock time; the whole days left over are the day offset. Two other modes work the same sum backwards: “Start time” tells you when to start a timer so that it finishes at a set time, and “Timer length” tells you what to set the timer to between two clock times, treating a finish that is earlier on the clock as the next day. Under More options you can repeat the timer for several rounds with breaks in between (Pomodoro sessions, laundry loads, batches of baking) and see a round-by-round schedule, set a reminder a few minutes before the end, switch to the 24-hour clock and pick the date. A checkpoint table lists the halfway point and the 10, 5 and 1 minute marks.

Worked example

A 90-minute timer started at 9:00 PM: 9:00 PM is 75,600 seconds after midnight and 90 minutes is 5,400 seconds, so the timer ends at 81,000 seconds, which is 10:30:00 PM the same day. The halfway point is 9:45 PM. Start the same timer at 11:15 PM and it ends at 12:45:00 AM the next day. Four 25-minute Pomodoro rounds with 5-minute breaks, started at 9:00 AM, take 115 minutes and end at 10:55 AM. Working backwards, to have bread out of the oven at 6:30 PM with a 1 hour 15 minute bake, start the timer at 5:15 PM.

Frequently asked questions