Clock synchronization tools are designed to calculate the differences between one's Mac clock and the one displayed by a reliable time server.
Consequently, such apps will correct the computer's system clock so that it matches the correct time displayed by the time server.
Some of these applications can also periodically control the time offset between the computer and a selected server, then show a notification with the verification's results.
There are some tools that also manage the daylight savings so that the user does wll have to modify anything.
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