Time Machine is an archive rather than a backup, because it retains multiple versions of files as you change them. That’s useful, because if you’re editing an image, a word-processing file, or another document, you can use Time Machine to restore a version before you made certain changes.
It always maintains at least one copy of every file, no matter how many times it’s revised, and if Time Machine has just a single version, it’s always the latest copy. If a file is never changed from its initial backup, Time Machine never deletes it, either.
Apple has Time Machine automatically consolidate snapshots. It’s set to run every hour, making a copy of any file changed from the previous 24 hours. After 24 hours, Time Machine deletes hourly backups, but keeps one per day. After a month, it deletes daily backups, but retains one weekly snapshot.
If Time Machine’s disk starts to fill up, macOS automatically deletes the oldest snapshots. The same is true with Time Capsule’s operating system for its internal and connected storage.
But you can always restore the exact current state of your drive as of the last hourly backup, no matter how many old snapshots are deleted.
I can explain this a little more easily with examples. Let’s say you plug in a drive and make it a Time Machine destination. MacOS immediately makes a full backup of the volumes you have set to archive via Time Machine. Once that’s complete, it then only makes incremental changes, adding versions of files that have changed.
If the initial backup contains, for example, a photograph in your Photos library that never changes, that copy is always retained, no matter how Time Machine culls updates after that point.
If you modify a Pages file, and have versions A (original), B, C, D, E, F, and G, Time Machine may delete versions B, D, and F as it consolidates hourly and daily backups. But you’ll still have multiple versions across time.
If Time Machine runs tight on storage, it’s possible that all backups except for version G could be deleted, but G will always be retained.
This Mac 911 article is in response to a question submitted by Macworld reader Jerry.
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Author: Glenn Fleishman, Senior Contributor
Glenn Fleishman’s most recent books include Take Control of iOS and iPadOS Privacy and Security, Take Control of Calendar and Reminders, and Take Control of Securing Your Mac. In his spare time, he writes about printing and type history. He’s a senior contributor to Macworld, where he writes Mac 911.