The Kindle WiFi system clock is set every time one syncs. One should sync before and after use to keep highlights and notes stored at the user Amazon Kindle site shown here:
https://kindle.amazon.com/your_reading
There is a manual clock also on the Kindle WiFi which can be set under Settings off the Main Menu on Page 2.
Pressing "Menu" brings up the manual clock at the top of the screen. Typing "Date" at the main menu and pressing the 5-Way key brings up the system time and date.