The D500X has a small back-up battery built in, which keeps the real-time clock running when the detector is not run from its regular batteries. If the D500X loses the time and date, the back-up battery is empty and should be replaced. The way this is done, depends on the version of the D500X. Early units require that the unit is disassembled while on more recent units, the back-up battery can be accessed after removing only the front panel (the one with the CF card slots). Please contact Pettersson Elektronik for more information about this.
A temporary solution is to avoid turning the D500X off with the INT/EXT power switch (use the 1/0 switch on the keyboard instead). Then the real-time clock will be powered from the main batteries instead.