Shortly after I got the 159, the Sat Nav unit had to be changed as the old one wouldn't read the disc, so my 'new' one is a new one if you get my drift.
The display on the original (2006 model year) unit was mostly in shades of grey when in radio mode but the new one (2008) is in shades of blue with orange highlights and has a slightly different initial menu page. I didn't have the original long enough to have the need to change the time, but hopefully the process isn't too different to what I do now and so this helps whichever one you have.
Ignition fob in (but don't start the engine) to switch on the unit.
When the initial start-up menu displays, turn the right hand control knob on the unit until 'Set Up' is highlighted and press the right hand control knob to accept.
If you look along the bottom of this next display screen, it should read 'Date & Time' above buttons 4,5 and 6. Press any one of these.
On this next screen, at the right hand side, there is a choice of 'Date & Time' and 'OK'. If necessary, turn the right hand control knob so that 'Date & Time' is highlighted and then press this knob to accept.
The highlighted tab should now jump to 'Day' in the main body of the display. Press the right hand knob again to accept and the highlight will move to 'Month'.
Repeat this process until the highlight reaches 'Hour'. Turn the right hand knob to alter the hour then press the knob to accept and the highlight will now move to 'Min'. Press again to move to 'Mode' and again and it will go back to the right-hand 'Date & Time' tab that we started at.
Turn the right hand knob to highlight 'OK' and then press the knob to accept.
The screen will return to the previous Set Up menu.
Press the main 'Audio Mute' button at the left hand side of the unit to return to radio mode.
Jobs a goodun! Hopefully.