Device: ASUS MemoPad 7 (K013), running the latest ASUS ROM based on Android Lollipop 5.0 (updated automatically with the included ASUS updater).
When I lock the screen, the system time stops ticking until I unlock it again. After unlocking, it does not update to current time, instead lags behind for the amount of time it was locked.
The time zone is set correctly and the device has stable Internet connection via Wi-Fi (no packet data option available on this device).
The date/time update setting is set to "Use network time", which disables manual time adjustment. After unchecking "Use network time" I can set the time manually.
Interestingly enough, after adjusting the time and re-enabling "Use network time", the time instantly skips back to the "lagged" value (how it was before manual adjustment).
Rebooting the tablet forces a time update, although the problem keeps persisting on screen lock, which renders the system clock, and thus the device, useless for my purposes.
A similar problem used to occur for me on a different device, but the time was updating automatically a couple seconds after unlocking. This is not the case here.