It has become almost de rigeur in quantum gravity to question the meaning of time in the Planck regime, before the universe was 10-43s old. As Henrik Zinkernagel and Svend Rugh point out, however, there are serious doubts whether our concept of time can be applied before the electroweak phase transition, when the universe was 10-11s old. The basic problem can be put like this:
1) Massless particles have no proper time. Massless particles trace null curves in space-time, and are invariant under conformal transformations (i.e., they are invariant under local scale transformations).
2) Before the electroweak phase transition, the weak and electroweak forces were unified, the electroweak Higgs field(s) were zero, and as a consequence, all particles were massless. According to the Standard Model, the elementary quarks and leptons, and the interaction carrieres of the weak force, only possess mass today by virtue of non-zero Higgs field(s).
3) Hence, before the electroweak phase transition, there was no meaningful measure of time.
I think Zinkernagel and Rugh have a point...