If you don't have access to the GUI apps to set the date, such as during an installation, it can be useful to set it using the terminal. This is easy as long as you remember the (somewhat unusual) order of the components of the date and time.
Use this command: date MMddhhmmyy
Where: MM is the month number dd is the day number hh is the hours (in 24 hour format) mm is minutes yy is the year (use yyyy if you prefer)