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There are different models of the Kindle DX (all are 3G only), not all using the AT&T network.
Please post (only) the first four characters of the device serial number (that is the encoded model number).
With that information, we can tell you if it is or is not a user of the AT&T network.
The search bar commands are Amazon provided features, no jailbreak required.
The set time command is:
;setTime
or:
;st
The date, time entry format is: yyyy-mm-dd HH:MM
For example:
;st 2012-07-22 17:59
The Wiki link is:
https://wiki.mobileread.com/wiki/Kin..._functionality
The time and date entry format is also shown in the table at the above wiki link.