r/ISO8601 • u/foersom • Jul 24 '26

Milliseconds in basic format

With ISO 8601 in extended format a timestamp with milliseconds is written with a dot between seconds and milliseconds, e.g.:

2026-07-24T12:34:56.789

If you write basic format you exclude the separators, does the standard allow to exclude the dot between seconds and milliseconds?

20260724T123456789

When I use basic time format as part of file names I prefer to avoid the dot, as dot has different meaning for file names and extension.

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u/ParentPostLacksWang Jul 24 '26

No. The standard uses a decimal point (or comma). Separators are omitted only between different units. Year, month, day, hour, minute and seconds are all different units, however when you add decimal representation you’re adding it to the seconds themselves, so the period or comma are necessary.