How to use it
- Add and subtract durations: fill in days, hours, minutes and seconds on each row, choose + or −, and add rows as needed. Values over 59 are fine; 90 minutes becomes 1 hour 30 minutes.
- What time will it be?: enter a start time (9:30 PM or 21:30) and the hours and minutes to add or subtract; the result says if it lands on another day.
- Time between two dates and times: pick both; the result is the real time that passes, with a note when a daylight-saving change makes it differ from the clock difference.
How it calculates
Durations are converted to whole seconds, added as integers and converted back, so nothing is rounded. Clock arithmetic adds the duration to the seconds since midnight and counts how many times it passes midnight in either direction. For two dates and times, the calculator turns each into a real instant in your device’s time zone and subtracts them; it also computes the plain clock difference, and when the two disagree, a daylight-saving change lies between them.
What we tested
On October 14, 2026 we checked the page’s own functions: 2,000 random duration sums against integer arithmetic, 2,000 clock calculations (12- and 24-hour input, rolling over several days forward and back) against Python’s datetime, and 1,500 date-time pairs in each of four time zones against Python’s zoneinfo, which uses the IANA tz database: New York (216 of the pairs crossed a daylight-saving change), London (189), Lord Howe Island, whose clocks move by only 30 minutes (185), and Kolkata, which has no daylight saving. Every result matched. A version that ignored daylight saving failed hundreds of these cases. In Chromium: the default rows (2 h 45 m + 1 h 30 m − 20 m) showed 3 hours 55 minutes, 3:55:00 and 3.9167 hours, and adding a row of 1 day 30 seconds gave 1 day 3 hours 55 minutes 30 seconds; 9:30 PM plus 4 h 45 m showed 2:15 AM next day and 1:15 minus 26 hours showed 11:15 PM two days earlier; with the browser in New York, 10 PM March 7 to 6 AM March 8, 2026 showed 7 hours and explained the 1-hour spring-forward; June 1 8:00 to June 3 17:30 showed 2 days 9 hours 30 minutes with no note; the page fitted a 390 px phone with no errors.
| Example | Result |
|---|---|
| 2 h 45 m + 1 h 30 m − 20 m | 3 hours 55 minutes (3:55:00, 3.9167 hours) |
| 9:30 PM + 4 h 45 m | 2:15 AM, next day |
| 10 PM Mar 7 to 6 AM Mar 8, 2026, New York | 7 hours (clocks show 8; they jumped forward at 2 AM) |
Frequently Asked Questions
Built by Derek Giordano · Part of Ultimate Design Tools