Irregular Time Machine backups are one of Sierra’s great mysteries. It now looks as if they might have been fixed in 10.12.4.
GCD
Schedule a periodic task as a Launch Agent. How close to the expected time does macOS actually run it?
This bug disrupts Time Machine backups and all the dozens of activities which macOS manages. It may make Sierra unsuitable for use in servers.
macOS Sierra doesn’t run periodic backups using a simple mechanism. Here’s the story in full detail.
Working out why Time Machine backups go wrong isn’t necessarily that different from unravelling classical mythology. In some ways.
The Eclectic Light Company 