The documentation points out that you can only edit the calendar as long as there are no events yet. This makes sense, as each event is linked to the calendar via a timestamp when it is created.
However, I have found a workaround that works well with Aeon Timeline 2. You can try it out with AT3, but make sure to back up your data first.
Here is how it works with Windows:
- In the Timeline view, select all events (Ctrl-A).
- Cut all events, moving them to the clipboard (Ctrl-X).
- Now change your calendar settings. Be sure not to copy/paste anything in the meantime.
- Paste the events back to the timeline (Ctrl-V).
Caution: After changing the length of e.g. a month, your events may be misplaced now because their timestamps refer to the old calendar. You may have to adjust them in bulk by selecting and dragging them to the right position on the timeline.
So only do this workaround if re-entering the events is more time-consuming than adjusting them on the timeline.
In any case, this workaround is very useful if you only want to change the language of the calendar or the names of the date/time units later on.
Update:
If you already have specified other items than events in your project, all this might not work as described. Then you can try to create a new project with the correct calendar, and copy/paste all elements there. However, relationships will break and have to be re-assigned.
See: