A message date displayed as 06/07 can be read differently by two colleagues. Thunderbird supports custom date-format preferences, documented from version 91, that can make its message-reader timestamp easier to describe consistently.

This is a display adjustment. It does not correct a sender's clock, change a message's recorded date, or select another time zone. If the reported event time itself is wrong, preserve that evidence and investigate it separately.

Record the baseline before changing two preferences

Choose a harmless retained message with a known date. Note its sender, subject, displayed timestamp and Thunderbird version. Keep a screenshot that excludes unrelated private messages so the same item can be compared after restarting.

Open Settings, General, then Config Editor at the bottom. Mozilla treats these advanced preference changes as unsupported and warns that future versions can change them. On a managed installation, follow the organisation's settings policy.

Search for intl.date_time.pattern_override.date_short. Record any existing value; if the preference is absent, create it as a String. Set its value to yyyy-MM-dd. Then set the String preference intl.date_time.pattern_override.time_short to HH:mm. Preserve the capital letters exactly.

Save unfinished work and restart Thunderbird for the date-format changes to take effect. These short-format preferences are used in the message reader; do not expect every date presentation elsewhere in the application to become identical.

Compare the same evidence after restart

Open the retained message again. Confirm that its identity is unchanged and that the date is now readable as year, month and day with the intended 24-hour time. A familiar afternoon timestamp makes a useful check that the time format is doing what you intended.

Compare a second message whose day is twelve or below. That is where a previously ambiguous numeric display is easiest to misread. Record the chosen presentation in the team's handover instructions if colleagues will quote it.

If the result is unsuitable, restore the previous values. Config Editor provides a revert control for changed defaults and a trash control for a newly added preference. Restart and inspect the same messages again. Keep this change limited to display formatting rather than editing other preferences in an attempt to make a questionable timestamp look plausible.

Sources: Thunderbird documentation, Thunderbird documentation.