How to Add and Subtract Time

Why Time Arithmetic Is Tricky

Unlike regular numbers, time uses a base-60 system for seconds and minutes, and a base-24 system for hours. That means 90 minutes is not 1.90 hours - it's 1 hour and 30 minutes. This mismatch is the root of most calculation errors.

Many people reach for a spreadsheet and convert to decimal hours (e.g. 1.5h), do the math, then convert back. It works, but it's slow and error-prone. A dedicated time calculator handles this automatically.

Adding Two Durations

The rule is simple: add each unit separately, then carry over when you exceed the unit's maximum.

Example: 2h 45m + 1h 30m

Minutes: 45 + 30 = 75 → carry 1 hour, keep 15 min Hours: 2 + 1 + 1 = 4h Result: 4h 15m

When working with seconds, the same carry rule applies: 60 seconds = 1 minute, 60 minutes = 1 hour.

Subtracting Durations

Subtraction works in reverse - you borrow from the next unit when the smaller unit goes negative.

Example: 3h 10m − 1h 45m

Minutes: 10 − 45 = −35 → borrow 1 hour (60 min), keep 25 min Hours: 3 − 1 − 1 = 1h Result: 1h 25m

Adding Multiple Durations (Totalling)

The most common real-world task is totalling a list of durations - for example, daily work logs or video clip lengths. The approach is the same: sum each column and carry over.

Example: 3 work sessions

1h 20m 2h 55m 0h 50m ────── Minutes: 20 + 55 + 50 = 125 → 2h 05m Hours: 1 + 2 + 0 + 2 = 5h Total: 5h 05m

Multiplying Time by a Number

Sometimes you need to multiply a duration - for example, if a task takes 1h 20m and you need to do it 3 times.

1h 20m × 3 = 80 min × 3 = 240 min = 4h 00m

Convert to the smallest unit (minutes or seconds), multiply, then convert back. TCalc.it does this automatically when you use the × key.

Is 15:03 a Clock Time or a Length?

Written down, 15:03 can mean three minutes past three in the afternoon or fifteen minutes and three seconds. TCalc.it reads a colon string as a time of day by default, and a bare 1730 as a plain number.

11:34:30 + 15:03 default: two times of day, which cannot be added as lengths: 11h 49m 33s

When a calculation fails for that reason, a hint appears over the keys explaining the reading and offering the other one. One tap recalculates the same expression. The switches also live in Settings (Video tab), and the Film profile has them on already, so subtitle and ffmpeg timings add up without touching anything.

Common Mistakes to Avoid

✗Adding decimal hours directly: 1.5 + 0.75 ≠ 2.25h in time notation. 1h30m + 0h45m = 2h15m.
✗Forgetting to carry: Writing 45m + 30m = 75m instead of 1h 15m.
✗Mixing units: Adding hours to minutes without converting (e.g., 2h + 90 ≠ 92).

Using TCalc.it for Time Addition

TCalc.it understands time notation natively. You can type 2h 45m + 1h 30m and press = to get 4h 15m - no conversion needed. The custom key row lets you insert unit labels (h, m, s) with a single tap, making it much faster than a standard calculator.