Is it possible to change the formatting of the calendar timeline time from the default 24 hour clock, to a 12 hour AM/PM format. I have made editing changes to the Time Format and Date Time format section through the Admin Console, however, these changes only affect the time and date format of users that make posts or update events that show up on the dashboard or other spaces.
If you mean the date/time picker calendar then
System > General Configuration > Advanced ,which you can refer https://confluence.atlassian.com/display/JIRA/Configuring+Advanced+Settings and follow the configuration in KB https://confluence.atlassian.com/display/JIRA/How+to+change+the+Date+Time+Range+Picker+format+to+24h+format
The same applies, the KB highlight what can be changed. It links to another useful KB article (https://confluence.atlassian.com/jira/changing-the-due-date-input-format-192536.html) which has an example of how to go AM/PM 12h format at the bottom of the page.
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