Check your manual. I don't see any battery but time is lost if there is a power interruption. Reset it manually and make sure the printer is always plugged into a live outlet. Most printers now don't get turned off. They go to sleep and are always on standby. 

https://www.dell.com/support/home/us/en/04/product-support/product/dell-2335dn/manuals

I had the same problem, guessed there was a battery somewhere, it took some time to find and replace it. There's a 3 Volt lithium 2032 battery on the control board.

I looked at the service manual and it seemed to say to remove the rear cover before removing the left side cover. I removed the rear cover (four screws) then the left side cover - service manual says three clips on bottom but doesn't mention the two screws on the front (access by opening the front door). On my control board, the battery was at the bottom. Replaced it and now it remembers the time. Leaving the printer connected to power didn't appear to be a fix on this generation of device.