Nokia Lumia 1520 - Why do the date and time get reset by themselves in my Lumia phone?

The date and time will change to the default values if you reset the phone by pressing down the power button for around 10 seconds. If the automatic date and time setting is not supported by your operator, you need to set the correct date and time manually.

To avoid the date and time getting reset by themselves, take care that the power button doesn't get accidentally pressed down for more than 10 seconds.

When the date is wrong, you may not be able to download the apps from the Store.