The process involves careful precision and split-second timing from the clockmakers. As well as re-setting the time, it gives them an opportunity to make close inspection of the clock mechanism as part of a rolling maintenance programme.
In addition to ensuring the most famous clock in the world is correctly re-set, the clockmakers have 2000 other clocks throughout the Palace of Westminster and the parliamentary buildings to adjust over time change weekend.
Time change schedule
This timeline provides a general guide to the clockmakers' schedule on time change weekend.
Timings are subject to change depending on the clock mechanism’s inspection and maintenance programme.
21.05 (BST): The hour strike is locked off Big Ben, the Great Bell. It is re-set for the strike at 02.00 (GMT). The hour bell is silent until then.
21.46 (BST): The quarter chimes are also locked off. This stops them chiming while the clock is being inspected.
21.48 (BST): The dial lights are switched off. This allows the clock hands to be advanced without drawing too much attention or causing public concern.
21.50 (BST): The clock hands are advanced to 12 midnight by releasing the double three-legged gravity escapement. The escapement regulates the clock's accuracy.
Releasing the escapement allows the weight which drives the clock mechanism via the going train to rotate freely. This advances the hands forwards. The process takes two minutes.
At this stage, great care must be taken to make sure the clock hands do not overrun past 12. If this happens the hands would have to be advanced right around until they reached 12 again.
21.52-23.00 (BST): The going train is stopped. This time frame is used to carry out inspections as part of a rolling maintenance programme.
00.00 GMT (01.00 BST): At the new midnight, the going train is started. The clock then runs for the next two hours - without the hour strike or quarter chimes. This period allows the clockmakers to monitor the clock's accuracy and make any adjustments to get it within one second of the new time.
01.46 (GMT): The hour strike and the quarter chimes are unlocked. This allows the hammers of the hour bell and quarter bells to operate at the next strike.
01.58 (GMT): The dial lights are switched back on in the following sequence: West, South, East, North.
02.00 (GMT): The quarter bells sound the Westminster Chimes, the Great Bell strikes the hour and the time change weekend procedure is complete.