As far as I know printers do not have a battery (or capacitor) to maintain date and time settings. The settings in MFC type printers that are usually maintained are stored in non volatile memory. Your printer for instance allows you to store names, telephone numbers, and fax data in its memory when the power is turned off.
If your printer is anything like the Brother printer that I have, it is probably been designed to never be turned off at the power outlet. Maybe yours too has been setup so that you should use the power off mode. (Press power button to turn it off only, do not turn off power outlet)
The benefit of using this mode rather than switching off the printer and turning off the power at the outlet, in the case of the Brother printer, is that besides maintaining the Date and Time setting, it periodically 'wakes up' and does a quick shortened version of a printhead clean, thereby maintaining the ink flow to and in the printhead and not allowing it to dry out and become clogged. I have "lost" a previous printer due to the fact that the printhead was totally clogged with dry ink due to not using it often enough and having the power off.
You have to weigh the benefits of turning off the power outlet against just turning off the printer.
Printer off Power outlet off
Pro : Power saved. Less ink wasted.
Con : Printhead may become clogged with dry ink if not used regularly. May waste ink doing multiple printhead cleans to get printer to print correctly after long period of inactivity. Lose Date and time setting.
Printer off Power outlet on
Pro : Printer will work first time after long period of inactivity. No printhead issues due to 'clogged' printhead caused by dry ink. Date and time setting maintained, except if there is a power failure of course.
Con: Power wasted. Ink wasted (this is debatable as to how much compared to doing multiple cleans to get it going if using the other option)
These are the specifications for your printer regarding power consumption.
Power consumption (Approximate)
Standalone copying mode: 17 W (ISO/IEC24712)
Ready mode: 7.5 W
Sleep mode: 3.9 W
Power off mode: 0.2 W