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So Time Machine keeps hourly backups for a day and dailies for a week and weeks until disk full. The problem is that I am paranoid and concerned about discovering that I made changes to a file over time in the past and that I want the version that disappeared between weekly backups. (Yes, yes, I use git and other version control solutions for many things but I like to be paranoid and hate things I can't configure myself).

Is it possible to change how long hourly and daily backups are kept for and if so where?

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If you want control over the retention policy, Time Machine isn't really the right tool for you; it's really designed to avoid requiring the user make decisions, and as a result it's not a good choice for someone who wants to be able to make decisions. The retention policy definitely follows this principle; there's no way (at least as far as I've ever seen) to adjust its retention policy.

In fact, there's not even any way to guarantee snapshots will be retained at all. If Time Machine decides it needs the entire backup capacity for the current backup (say, because you just connected your big external disk full of videos and it wants to back that up -- this happened to a friend of mine), it'll actually remove all old snapshots to make room. Note that there is an option in the preference pane to notify you after old backups are deleted.

Basically, Time Machine places priority on getting the current data backed up vs. retaining old data, and there's no way to change this. (It is, after all, a backup system, not an archiving system.) If your priorities differ, you really should find another archiving tool.

(BTW, you did misread its normal policy slightly: it keeps the daily snapshots for a month, and only after a month does it thin them down to one a week.)

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