there is something called "nitz
" in GSM - have a look in the documetation for your shield on how to access this data. However, some caveats:
a) not all carriers supply this, and which bits of information you get can be different (e.g. DST)
b) the resolution is not great ("in the order of minutes"), and IIRC the time is sent onyl once when you log in to a cell, so it could be out of date when you look it up (again, see the codumentation for your shield)
Other options would be to get the time from a simple http header, or send yourself an SMS and parse the datetime from its metadata.