How this instrument works
This is the inverse of an ordinary clock-duration subtraction: instead of asking "what's the gap between two times," it asks "if something starts now and takes this long, what time does it finish" — the everyday math behind setting a timer, estimating an arrival, or figuring out when a shift or a recipe will actually be done.
It works in minutes-since-midnight underneath. The start time converts to minutes past 00:00, the duration (in hours and minutes) adds directly to that figure, and the sum is taken modulo 1,440 — the number of minutes in a day — to land on the resulting clock time. Whatever whole days that sum passed through along the way is reported separately as "days later," since a long enough duration can roll past midnight more than once.
It's the practical tool for anything where a known starting point and a duration are given but the finish isn't — a flight's departure plus its flight duration, a shift's start plus its length, a recipe's start plus its total cook duration, or an appointment's start plus its expected length, whenever the answer needs to land on an actual clock reading rather than just a duration.
- Enter the start hour and minute, using the 24-hour clock.
- Enter the duration's hours and minutes.
- Read the resulting end hour and minute.
- Check "days later" — 0 means it finishes the same day; 1 or more means it rolled past midnight.
Worked example — a late flight landing after midnight
A flight departs at 22:30 with a 3-hour-45-minute flight time: start = 22×60+30 = 1,350 minutes past midnight; duration = 3×60+45 = 225 minutes. Total = 1,350 + 225 = 1,575 minutes.
1,575 mod 1,440 = 135 minutes, which converts to 02:15 — so the flight lands at 2:15 AM. Days later = floor(1,575 ÷ 1,440) = 1, confirming the landing happens the calendar day after departure, exactly the kind of overnight detail that's easy to miss when just adding "3h45m" to "22:30" in your head.
Questions
What does the "days later" figure actually mean?
It tells you how many full calendar days the start time plus duration rolled past before landing on the displayed end clock time. A value of 0 means the end time falls on the same day as the start; a value of 1 means it rolled past one midnight (the next day); a value of 2 or more means the duration was long enough to roll past midnight multiple times, which only happens with durations well over 24 hours.
Can the duration be longer than 24 hours?
Yes — enter any number of hours, including well over 24, and the calculator still resolves correctly to a valid clock time plus however many days later it landed. A 25-hour duration starting at midnight, for instance, lands at 01:00 the next day (1 day later), since 25 hours is exactly one full day plus one extra hour.
Does this accept 12-hour AM/PM start times?
No — the start hour field expects 24-hour (military) format, 0 through 23. Convert an AM/PM start time first: hours from 1 AM to 11 AM stay the same number, 12 PM (noon) becomes 12, and 1 PM through 11 PM become 13 through 23.
How is this different from the Clock Duration calculator?
They solve opposite problems. Clock Duration takes a start time and an end time and computes the gap between them (the duration). This calculator takes a start time and a duration and computes the resulting end time — the reverse operation, useful whenever you know how long something takes but need to find out when it finishes rather than how long a known start-to-end span lasted.
Can I chain multiple durations together, like several connecting flights?
Not in a single pass — run the calculator once per leg, using each result's end time (and noting any days-later rollover) as the next leg's start time. Add up the days-later figures across all legs to track the total calendar days elapsed by the time the final leg's end time is reached.