What AT command should be sent to modem to get time?
I have used the following command:

AT+CCLK?


which returns the date and time, like:

+CCLK: "00/01/12,05:44:53+00" 


successfully in products using three different cell modules: Enfora, Telit, and Sierra.  The fact that it uses a + character after the AT implies it is a common (as opposed to proprietary) command, and should be available on most if not all cell modules.  (Proprietary commands use a vendor specific character after the AT, like AT$, AT%, AT! etc.)

As I mentioned in a comment, if you can't get the time using this command (for example, it returns all 0's), then another way is to connect to the network and get the time off of a server using an HTTP command.

For example, the following HTTP request:
HEAD /index.html HTTP/1.1\r\n
Host: www.somehost.com\r\n;
\r\n


where somehost is either your own server or Google.com etc. This also assumes index.html is the main page for the site (not the case for Google).

It will return a header like:

HTTP/1.x 200 OK  
Transfer-Encoding: (something)  
Date: Fri, 13 Nov 2013 04:36:25 GMT
(lots more stuff)


As far as I know, all severs are obligated to return GMT time in their headers.