Your Kindle Fire is automatically configured to sync current date and time from Amazon's servers (it is factory-setup with your username, and they know your shipping address, thus time zone!)
To change your date, time, or time zone, tap on the icon (top right), and pick "More..."
Then tap "Date & Time" (third visible option from bottom). The next option screen shows "Automatic - Use network provided date & time" set to On. Tap the switch to turn it Off and manually adjust:
Tap on "Set Time" and the plus and minus buttons to change hours and minutes; tap on AM/PM to toggle and add/remove 12 hours. Military time (24-hours time) isn't currently supported, at least for Kindles sold in the US or Canada. Tap Save to accept the new time:
Choose "Set Date" for a different day/month/year with the plus and minus buttons, and tap Save:
Tap on "Select Time Zone" to change it (refer to the table below for US/Canada time zones to GMT conversion). Tapping to pick a zone will automatically apply and save.