GSM network time synchronization

When I setup M95 modem to synchronize clock with the cellular network, the clock is not set up correctly:


Then I do:

+QLTS: “19/10/08,12:11:49+08,1”
+CCLK: “19/10/08,13:11:54+08”

The QLTS hour is correct, but modem clock is set 1 hour later.
I have seen this happen also with other modem models (BG96) and also with AT+QNTP.

Does anyone know why that happens?

I did some tests with CTZU=1 =2 and =3
My previous results were bogus, because the RTC clock is not updated immediately after QLTS reports the new time. Go figure why!

So now I confirmed: the following:

  • CTZU=1 is correct
  • CTZU=2 is off by 1 hour
  • CTZU=3 is off by 2 hours

Here in Italy we are in Time Zone +01hr and we have DST, so we are 2hr later than GMT, or better UTC.
So what happens is that the modem sets the hour according to CTZU mode setting, but does not change the TZ offset accordingly, so it’s impossible to tell what time the RTC clock is using. If you assume the time format of CCLK is GMT “hh:mm:ss±zz” (as it is documented) you get the wrong time!

Dear Sir,
Could you try to follow up the following steps to confirm GSM network time synchronization? Thanks!

Thank you Kyson, I didn’t know I had to reset the modem for the time synchronization settings to take effect.

Anyway, you just proved what I said:
QNITZ reports time as 07:09:54+32 (that is 7 hour GMT +8 hours timezone)
Afterwards the RTC clock is set to 15:09:54+32 (that is 15 hour GMT +8 hours timezone)

The RTC clock has been updated to the local time (7+8=15) but the timezone offset is still there. This is confusing. How do you know the RTC clock is using local time if you keep the timezone offset? To interpret CCLK result correctly you have to know it’s using local time and then the time format is something like LOCAL time + TZ offset.

In order to avoid confusion, I will use CTZU=1, as that seems to keep the time received by the GSM network unchanged, which is correctly in the expected format (GMT time + TZ offset).