How to sync past events in your Calendar on your iPhone and Mac

To keep a proper record of your calendar, you need to adjust a setting in iOS.

Apple
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Elliott updates past events in his calendar when his itinerary or other details change. However, during a recent two-week trip:
I was in a country where there were no Internet connections. When I came home, I made the correction on my iMac because I wanted to have a correct record of the itinerary changes. I notice all my devices would not update because it was a past event.

iOS lets you change the look-back sync duration.
There’s no setting on a Mac that helps with this, and no way to determine how old events are updated. However, in iOS, you can choose how far back events synchronize. In Settings > Calendar > Sync, you can pick either all events or a duration back two weeks, one month, three months, or six months.
Apple offers no support notes on this, nor, in my testing, can I figure out if it relates to just syncing this events once or when past events change.
What I suspect happened to Elliott is that his iPhone is set to Events 2 Weeks Back, which appears to be the default, and thus when he made the change so his Mac after two weeks away, those older events never synced back. Changed the setting in iOS should propagate those changes.
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