Currently date/time format settings are in multiple places. This can be quite confusing as you need to change format in different places using different formatting strings.

To change the displayed date/time you need to goto Look and Feel( https://<instance.atlassian.net>/secure/admin/LookAndFeel!default.jspa ).

This doesn't change the date/time pickers formats. To do this you need to go into Advanced settings( https://<instance.atlassian.net>/secure/admin/AdvancedApplicationProperties.jspa ) and then select date time picker "jira.date.time.picker.javascript.format" or date picker "jira.date.picker.javascript.format". Also this is in different formats to the above look and feel.

Suggestion: Consolidate date/time configuration in once place and use the consistent  date/time formats 

Notes

The format of most date / times in Jira Cloud is determined by each user's Language selection, here: https://id.atlassian.com/manage-profile/account-preferences

This is documented further here: Manage your language preferences