The format of most date / time fields in Jira Cloud issues is currently set per user depending on the Language they select on https://id.atlassian.com/manage-profile/account-preferences. This is documented here: How do date and time formats work in the issue view?

We should be able to set the date / time format independent of the selected language preference.

Related  requests

• There are some fields that still don't change format depending on one's language setting. We have created separate feature requests for this e.g. JRACLOUD-81778 The "Added date" format in the Attachments section on issue view should respect the user's language settings
• JRACLOUD-76814 – Allow further customization for date and date/time field (that's not limited to Account preferences)