Thunderbird email--date formatting guide

2026-02-05

go from default to shortened ISO (or any kind)

this is a quick guide explaining Mozilla's poor documentation of the feature (half their page references a preference section that doesn't exist).

applicable since Thunderbird 91, using version 140.7.0esr as of writing.

TL;DR

• go to general settings
• search config editor
• add custom strings with new values:

intl.date_time.pattern_override.date_short

value = yyMMdd (for shortened ISO)

intl.date_time.pattern_override.time_short

value = HHmm (for 24H clock)

intl.date_time.pattern_override.connector_short

value = {1}-{2} (for date-time separator)

i don't get it, let me see

• open the general settings page (not account settings)
• via top right, "Find in Settings", search config editor
• a button for "config settings" should appear. click it.

• now on "Advanced Preferences" tab, in "Search preference name" box:

1. paste: intl.date_time.pattern_override.date_short
2. check the "String" box
3. click the "+" button

• input value: yyMMdd (feel free to add dashes or use "yyyy" for all digits)
• hit enter, or click the checkbox

repeat this process for the next two custom preferences

intl.date_time.pattern_override.time_short

value = HHmm (feel free to use a colon)

intl.date_time.pattern_override.connector_short

value = {1}-{2} (feel free to use space, underscore, etc.)

if you want to use other alternate formats, reference the Unicode Date Field Symbol Table that Mozilla links to!