If you lock the desktop, the clock on the lock screen will not update after a minute or two of inactivity.
So for example if you lock your screen at 12:00 and go to lunch when you return an hour and a half later your lock screen clock will show something like 12:01 or 12:02.
The screen will immediately update as soon as you press a key or move the mouse.

Display powering off when idle on lock screen on a Win 10 machine is controlled by a timeout value called "Console lock display off timeout" which may default to 1 minute and is hidden. Set high value of the above timeout. Set to 0 or to a high value (eg.: 100)

Control Panel location:
Set the auto turn off display time value to 0 from control panel >Power Options > Advanced Power Settings>Display> Turn of the display, set it to: 0 Min.

Group policy location:
Computer Configuration - Administrative Templates - System - Power Management - Video and Display settings - Turn off the display - enabled + 0 seconds

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Problem Cause

System default console lock time out is 1 minute