Interesting approach are [casual sets][1] together with [causality relation][2]. There are physical models of quantum gravity based on such approach for example [casual dynamic triangulations][3] invented by Renate Loll, Jan Ambjørn and Jerzy Jurkiewicz. In this theories time is an order defined by causality of events which is base notion and spacetime structure follows from that in dynamic way. 


  [1]: http://en.wikipedia.org/wiki/Causal_sets
  [2]: http://en.wikipedia.org/wiki/Causal_structure
  [3]: http://en.wikipedia.org/wiki/Causal_dynamical_triangulation