How to Make Sure SIM900 modem will always give me +PSSUTTZ string?
One way to get current time is by sending a message to itself. Below is an example:

AT+CMGF=1 
AT+CSCS="GSM" 
AT+CMGS="0123456789" your phone number
THIS IS TEST         test message
...
...
+CMTI: "SM",1 recevied a message at index:1
...
...
AT+CMGR=1
+CMGR: "REC UNREAD","0123456789","","14/11/15,15:03:13+22" got current time
THIS IS TEST

Sync your SIM900 with current time by
AT+CCLK="14/11/15,15:06:45+22"

For commands refer datasheet.