Sometimes Apple needs your Mac to know exactly what time it is. Most people leave their Macs set to the default setting in which the time is set via a network server that keeps it up to date down to the second. But you may have turned that feature off, reset the clock manually to check something or prevent a date-based expiration, or have a hardware problem that’s causing a glitch. (Some Macs include long-lived battery cells designed to keep the clock ticking and preserve other settings.)
If your Mac doesn’t know the proper time, you may have trouble with your Apple ID and iCloud syncing, be unable to log in to the App Store or download updates, have your Mac say it can’t connect to servers when in macOS Recovery, or have the process fail for recovering a lost FileVault key when you’ve allowed it to be stored in your iCloud account.
If your Mac is up and running, use the Date & Time preference pane to check the time. You can enable automatic date/time setting if it’s off, or if it’s on and incorrect, disable it and set the clock manually. (If you unplug your desktop Mac and plug it back in and the clock is incorrect, you almost certainly need to replace its internal settings battery.)
Read more at MacWorld.com