firmware: RTC clock is not precise

[pat1]

The time go in sync with NTP:

dom 5 dic 2021, 23:45:09, CET #2021-12-05T22:45:00 N: SIM800 UDP status... [ OK ] [ CONNECT ]
dom 5 dic 2021, 23:45:09, CET #2021-12-05T22:45:00 N: NTP send request success
dom 5 dic 2021, 23:45:09, CET #2021-12-05T22:45:00 N: NTP response... [ OK ]
dom 5 dic 2021, 23:45:09, CET #2021-12-05T22:45:09 N: RTC set... [ OK ]
dom 5 dic 2021, 23:45:09, CET #2021-12-05T22:45:09 N: Current System date and time: 5/12/2021 22:45:9
dom 5 dic 2021, 23:45:09, CET #2021-12-05T22:45:09 N: Current date and time is: 5/12/2021 22:45:9
dom 5 dic 2021, 23:45:09, CET #2021-12-05T22:45:09 N: Acquisition scheduling...
dom 5 dic 2021, 23:45:09, CET #2021-12-05T22:45:09 N: --> observations every 1 minutes
dom 5 dic 2021, 23:45:09, CET #2021-12-05T22:45:09 N: --> report every 3 minutes
dom 5 dic 2021, 23:45:09, CET #2021-12-05T22:45:09 N: --> starting at: 22:48:0
dom 5 dic 2021, 23:45:09, CET #2021-12-05T22:45:09 N: --> testing at: 22:46:0

but after ~9 hours :

lun 6 dic 2021, 08:58:04, CET #2021-12-06T07:58:00 N: [Test] I2C-ITH-35 Values: 29277 38 -1	Json: {"B12101":29276,"B13003":38}

the first time in log come from PC, the time after "#" from Stima and are 4 seconds shifted.