That is the classic symptom of a CMOS battery that is failing. However keep in mind that at 3 years old the main battery is probably getting weaker and will need replacing in the not too distant future. Dell support places the main battery life in a laptop at 1 1/2 - 2 years old or so.
Hello, since about two weeks the device has enormous problems to start. Often, after pressing the on button, only the keyboard light turns on and the screen remains black. After trying several times, the notebook can usually be started after pressing the on button for about 40 seconds. Then usually comes a BIOS screen that some problems with the power supply have occurred ("time of day not set"). You can simply confirm this and then it starts normally. Interestingly, the time displayed in Windows after startup is almost always exactly a few minutes after the device was last turned off. The notebook is about 3 years old and is used excessively. Battery works perfectly. Is it possible that the CMOS battery is defective? Thanks for help!  
XPS 13 9370 starting problems