DIAGNOSIS Yes, the internal backup battery that maintains the clock and settings during power loss is depleted. This is a common failure point.
FIX The brewer needs to be opened to replace the battery. You must unplug the unit and remove the rear panel (a few Phillips screws). The battery is a standard 3V CR2032 coin cell holder on the main control board. Swap the old battery for a new one, reassemble, and plug in. The clock should now retain time after a brief power outage.
IF THAT DIDN'T WORK If the clock still resets after a confirmed battery replacement, the main control board itself may have a fault and would require replacement—contact Ninja Customer Service at this point.
HEADS UP When the backup battery is dead, all your custom settings (like favorite brew strengths, My Brew profiles, and Wi-Fi connection) will also be lost every time you unplug it, not just the clock.