Or as has been recently discovered, the speed of light is not the absolute that Einstien predicted. Tests made in the new particle accellerator have demonstrated faster than light speeds.
If God designed the cosmos so that light appeared when created, it would seem as if light had been streaming for billions of light years before it got to us, but if God just "turned on the lights" it would be here then.
An example of this for us (though obviously our scale is far to short to comprehend the issue) is what happens when we shine a flashlight or laser into the night. Do we see a stream of light advancing as it goes deeper into the darkness or do we see a stream that goes on as far as it goes on? This has been an issue that has given cause for much experimentation over the years. Also, is light a wave or a particle? Appears to be both, yet our current state of physics stipulates that it cannot be both at once.
It appears that is just what happened, "Let there be light..." happened before the creation of the bodies that produced it. So, in essence, theories that depend on measuring the light then interpolating the dates back into time when that light was produced would give the impression of a cosmos more ancient that it may actually be.