With the exception of what are called the First Dog Watch and Last Dog Watch, these occur from 4pm to 8pm and are each of two hours' duration, the watches onboard ship are a four hours' duration, the passage of each half hour of which is marked by a sequence of 'one bell' to 'eight' bells.
In former times, before the introduction of mechanical timekeepers, time onboard ship was measured using a '30 minute' sound glass. Starting at the beginning of each watch it was the duty of the helmsman or quartermaster to reverse this glass every thirty minutes immediately the sand had run through the top-half to the bottom-half, and on each occasion he did this he would strike on the ship's bell the number of times he had turned the glass, giving double strokes for whole hours followed by a single stroke for the last half hour.
'Dog Watches' are used to alter the sequence of watches during the 24 hour period, without them watchkeepers would always be working the same watch sequence. This necessitates a change in the sequence of the usual 'one bell' (4.30pm) to 'four bells' (6pm). The 'Last Dog' begins at 6pm but at 6.30pm instead of 'five bells' being struck, only 'one bell' is struck and the sequence carries on until at 8pm, 'eight bell' is struck.
'Ship's Bell' clocks which only repeat the striking sequence for every four hour period starting from midnight are not uncommon, but it is particulary rare to have a clock which takes care of the 'Dog Watch' sequence. It can be seen that it is obvious that the hands of such a clock have to be set to show the correct half of each day to avoid the 'Dog Watch' sequence being struck between 4pm and 8pm and NOT between 4am and 8am.