The importance of the garden in this novel is that it represents the past and the present and the inevitable changing nature of time. Tom has to confront the fact that his garden in the past, which is such a big and wonderful place to play, is very different in the present, as he soon discovers when he rushes out to see it in the day time:
At the back of the house was a narrow, paved space enclosed by a wooden fence, with a gateway on to the sideroad at one end. There were five dustbins...
In the same way, the house, that appears to be so big and so dominant, in the present is split up into separate flats. The garden and the setting of this novel therefore serves to reinforce the theme about the ways in which time is inexorable and change happens. Although Tom is able to jump back in time through the magic of the clock, he still has to accept that time does change and move on. What does not change and what cannot be taken from him and Hetty is their enjoyment and pleasure of playing together.