Tuesday, October 29, 2019

Time Machine Beaten by the Clock

Howard Oakley:

Times here are given in UTC, which would a minute earlier have been an hour behind local time, so this backup would have actually started at 02:00:41 according to local clock time had the clocks not been changing that night. However, the clock change occurred at 02:00:00 local time, so for the second time that morning, the local time would have been showing 01:00:41, this time the same as UTC.

These entries show Time Machine successfully making its three snapshots, and naming each com.apple.TimeMachine.2019-10-27-010127, in accordance with both UTC and local time, the latter having just changed.


By an odd quirk of fate, this backup chose to try to create its three snapshots at 02:01:27. For a reason I can’t fathom, it decided to name those snapshots as if it was an hour ago, 01:01:27, which gave them identical names to those made during the last backup. Those snapshots failed because of the name conflict, and the whole backup was aborted as a result.

He disagrees, but from what I see—farther from the prime meridian—Time Machine names its snapshot folders using local time, not UTC. So, even during normal operation, it’s possible for two different instants in time to map to the same name.

I’d love to know why Apple doesn’t use UTC in the filesystem. The restoration UI would still have the opportunity to format the dates for display using local time.

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