  We investigated the information carried by the data of direct photons, ie,
the transverse momentum spectrum and the elliptic flow $v_{2}$ from Pb+Pb
collisions at $\sqrt{s_{NN}}$=2.76TeV measured at Large Hadron Collider (LHC)
and from Au+Au collisions at $\sqrt{s_{NN}}$= 200 GeV measured at Relativistic
Heavy Ion Collider (RHIC), in the frame work of (3+1)-dimensional ideal
hydrodynamical models constrained with hadronic data. We found those direct
photon data may serve as a useful clock at the early stage of heavy ion
collisions. The time scales of reaching thermal and chemical equilibrium,
extracted from those data, are about 1/3 and 1.5 fm/c, respectively. Thus the
large elliptic flow of direct photons is explainable. High order harmonics, ie,
$v_3$, $v_4$ and $v_5$, of direct photons from Pb+Pb collisions at 2.76TeV are
also predicted, as a further test to compete with those who claim new sources
of photons to account for the large elliptic flow of direct photons.
