How to Add and Subtract Time

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.

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

Example: 2h 45m + 1h 30m

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

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

Example: 3h 10m − 1h 45m

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

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

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

• ✗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).

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.