Duration Add/Subtract Calculator

Add or subtract ISO 8601 durations from a base time in any time zone, with millisecond precision.

Base time
Format: YYYY-MM-DD HH:mm:ss.SSS

Offset: UTC+00:00

Format: YYYY-MM-DD HH:mm:ss.SSS

Duration
Supports PnDTnHnMnS with optional milliseconds.

Supports PnDTnHnMnS with optional milliseconds.

Results

Add
Base time + Duration

Date & time

2024-01-01 13:00:00.000

ISO 8601

2024-01-01T13:00:00.000Z

Unix timestamp (ms)

1704114000000

Unix timestamp (s)

1704114000
Subtract
Base time − Duration

Date & time

2024-01-01 11:00:00.000

ISO 8601

2024-01-01T11:00:00.000Z

Unix timestamp (ms)

1704106800000

Unix timestamp (s)

1704106800

What is a duration?

A duration is an amount of time you add to or subtract from a base time. This calculator starts from a local date and time in a chosen time zone, then applies the same duration in both directions.

ISO 8601 examples

  • PT45M means 45 minutes.
  • P2DT6H means 2 days and 6 hours.
  • P1DT2H3M4.005S means 1 day, 2 hours, 3 minutes, and 4.005 seconds.

How this calculator works

  • Enter a base time in YYYY-MM-DD HH:mm:ss.SSS and choose the time zone you want to evaluate.
  • Enter the duration either as ISO 8601 text or with the day, hour, minute, second, and millisecond fields. The tool keeps both inputs in sync and normalizes overflow.
  • Enter positive durations only. Use the built-in add and subtract result cards to compare both directions.
  • The result cards show the adjusted local time, the UTC ISO 8601 timestamp, and Unix timestamps in seconds and milliseconds. Offsets can change around daylight saving time transitions.