Unambiguous date & time

If I gave you an assignment due on 3/4, would you complete it by March 4 or April 3? US Americans would assume March 4, but Europeans would assume April 3.

Communicating with a globally distributed team requires clarity. Fortunately, the International Organization for Standardization (ISO) provided a common date format to remove ambiguity in 1988.


The ISO date format is human readable and visually sortable with the components arranged largest to smallest.

Traditional US American

3/28/2023 14:56:30 PM

month day year hours minutes seconds

Traditional European

28.3.2023 14:56:30

day month year hours minutes seconds

ISO 8601 standard


year month day hours minutes seconds

Week numbers: 2023-W13

Week number dates are used commonly in manufacturing and by Europeans. They are indicated after a year with a W and a 2 digit week number.

I appreciate them as a progress indicator through the 52 weeks in a year.

Calendar apps typically have a way to show week numbers. Look for it under settings.

Keep in mind that ISO weeks start on Monday and week 1 of the year is the first week with a Thursday in it. Some years have 53 ISO weeks because Gregorian calendars do not evenly divide by 7.

Quarter numbers: 2023-Q1

This is non-standard, but still useful. Similar to week numbers, yearly quarter numbers can be indicated after a year with a Q and the quarter number.

Further reading for software developers


Time can be appended to a date. The standard provides increasing levels of optional specificity in the pattern of YYYY-MM-DDTHH:MM:SS.000.


  • 2023-03-28T14:56

    base time with hours and minutes

  • 2023-03-28T14:56:30

    with seconds

  • 2023-03-28T14:56:30.123

    with milliseconds

Time zones

Time zones are location and season dependent. However, time offset from Coordinated Universal Time (UTC) can be specified.

Examples of the same moment in time:

  • 2023-03-28T14:56Z

    Z = zero offset, a shortcut for UTC

  • 2023-03-28T14:56+00:00

    Dublin standard time. 0 offset is always positive.

  • 2023-03-28T06:56-08:00

    San Francisco standard time, negative 8 hours offset.

  • 2023-03-28T07:56-07:00

    San Francisco daylight saving time, negative 7 hours offset.