Can one extract time from GSM mobile data, without a SIM card?

[peter-h]

The above post seems to not make any sense.

[gargle]

just be careful, GPS and cell station(UTC) haven't the same time. there is several seconds between.

[voltsandjolts]

Your missing sumthing hear
The mobile device is getting its time signal from a base station that using a  GPS disciplined oscillator
so while your time is derived from a rubidium reference in the base station that reference is kept in sync with all the other base stations & UTC using the satellite navigation systems.

I'm guessing your point is 'you may as well use gps, because mobile time is gps derived anyway'.
However, the mobile time signal has much better availability inside buildings.
Or are you trying to make a different point?

[indeterminate]

I'm guessing your point is 'you may as well use gps, because mobile time is gps derived anyway'.

Yes
But you raise a good point about in building availability

[GromBeestje]

Also, the smart phone without sim card mentioned earlier, is it possible it grabs time from GPS in such case?

[krakatit]

Since the question says "extract a time" it is not clear what "time".
Without a SIM it is not easy, but if we allow a "time" to be a relative flow of oscillations it is possible to get precise timing from the VCXO of every GSM transceiver even without active SIM network registration. The 13 or 26 MHz VCXO is adjusted with every FCCH frame to a BTS GPS or atomic clock disciplined oscillator, the relative accuracy is +-50ppb (cca +-45Hz@900MHz, twice at the phone receiver). Short term variance (phase noise) is high but in a long term is well averaged (said to be < 10^-12), so MCU clock derived from this VCXO and local real time set once is pretty precise.

[voltsandjolts]

Since the question says "extract a time" it is not clear what "time".

It's quite clear if you read the OP, this topic is about real time, not just frequency.