Facepass 7 PRO changes its time ramdomly

Hello, we have 2 Facepass 7 PRO in a stand alone installation (we do not use Crosscheck software, instead a propietary software), both conected by wifi.

Totally we have 4 Facepass 7 PRO and 1 C2C PRO in the same installation.

The devices are working fine for 1 year, but about 3 weeks ago we are experiencing the following issue.

In one of them, the time changes ramdomly with 1 hour less, and then ramdomly again it changes for the right hour.

This device is configurated as the same than the other Facepass7Pro, it has DST set, no time zone set, no Group set and no schedule set.

Firmware ver:03.76.B6_dark_BT107
Kernel Ver. :g4aaa48c
File system ver. :01.76.02

In other Facepass7PRO device, with the same settings than before, this morning we have been noticed that it has the time 1999-11-30 00:28:46

Firmware ver:03.76.B6_dark_BT107
Kernel Ver. :g4aaa48c
File system ver. :01.76.02

I will appreciate any helping tips.

Thank you in advance.