Clocks.
WHY IT MATTERS
Certificates, authentication and logging all depend on accurate time.
WHAT GOES WRONG WITH A WRONG CLOCK
Certificate errors on sites that are fine Two-factor codes rejected Authentication failures Logs that cannot be correlated
THE TWO-FACTOR POINT
Time-based codes depend on the device clock.
A phone with drifted time produces codes that are rejected.
Frequently misdiagnosed as a wrong password.
HOW TIME IS SYNCHRONISED
Automatically, from time servers, on most devices.
WHAT TO CHECK
That automatic time is enabled That the time zone is correct
FOR SERVERS
Synchronisation configured and working.
WHAT TO CHECK WHEN CERTIFICATES FAIL INEXPLICABLY
The clock, before anything else.
WHAT ELSE DEPENDS ON IT
Scheduled tasks Backup timing Anything comparing timestamps
WHAT TO DO IF A DEVICE DRIFTS PERSISTENTLY
Investigate the hardware.
