A four-dimensional hypercube has 8 hypersurfaces that are three-dimensional cubes. The cross in Dali's painting represents these 8 cubes "laid out flat" in three dimensional space, in the same way that an ordinary cubic box can be undone and its surface (which consists of 6 squares) can be laid out flat in a plane.
Click here for a fine, big view of this painting.

Once there (it may take some time), you can look in the INDEX for "Persistence of Memory" with its flowing clocks, inspired perhaps by Einstein's relativity. There is also "Disintegration of the Persistence of Memory"....