Added 12h/24h switch and Standard / Daylight Savings Time Switch! See steps 7 & 8.
Typically computers get their internal clocks adjusted by connecting to a Network Time Server (Internet or Local), using the Network Time Protocol (NTP). That functionality is also available to microcontrollers like the Arduino. We have put together an inexpensive and easy to follow instructable that will allow you to add official time keeping to a project. Have fun!