The Time Apprentice by Val Tyler
When Old Father Tim, Greenwich's Guardian of time, feels a wobble in the earth's floor, he isn't overly concerned time has slight wibbles now and then. He has allowed for this. But then two events occur in this magical invisible world that cause quakes so massive that human and Guardian time are shaken completely out of alignment, causing our worlds to drift further and further apart. Old Father Tim and the Guardians decided that the only way to stop this happening is to search for and repair the Gemetbur - a thing of legends and before time, set up to govern time before Guardians ever existed. But, when Tid and Sofi discover in a dusty old children's book that their grandfather, Old Father Tim, is in grave danger should he reach this legendary thing, they set out after him and everyone is in a race against time to save each other as well our worlds!