A tensed fact is a fact that is true in relation to present experience, but has no universal or permanent truth value. "Yesterday was July 26th", "today is Wednesday", or "I got a new job last month" are all tensed facts.
Some people believe that God must know tensed facts to be omniscient. William Lane Craig formulates a hypothetical argument that God is not timeless as follows:
1. A temporal world exists.
2. God is omniscient.
3. If a temporal world exists, then if God is omniscient, God knows tensed facts.
4. If God is timeless, He does not know tensed facts.
5. Therefore, God is not timeless.
There are, however, immense problems with this argument, regardless of whether or not we ultimately conclude that God is timeless.
Tensed facts only exist relatively by definition: a tensed fact is relative to the temporal moment as experienced by the specific observer -- as the momentary experience changes, so may the accuracy of the tensed fact. Further, a tensed fact (such as "it is Wednesday") may or may not be "true" for a separate observer (such as someone on the other side of the world).
Since there is by definition no universal and permanent truth or falsehood in tensed facts, God certainly does not know the universal and permanent truth or falsehood of any tensed fact -- it doesn't exist to be known. We could just as easily argue that since God does not know "the smell of the number red", He is not omniscient. But this is utter nonsense! In reality, though, such a nonsensical argument is no more absurd than the one offered above for God's temporality! In light of this, (4) above should be rewritten as such:
4. If God is timeless, there are no tensed facts relative to Him for Him to know.
However, God is well aware that as we perceive Wednesday, then for us, the statement "it is Wednesday" is a true tensed fact. We should then also take an eraser to (3) and edit it to make it more clear:
3. If a temporal world exists, then if God is omniscient, God knows all tensed facts as perceived by others, given specific conditions.
Thus, God knows all that we know, including tensed facts relative to us, in spite of the fact that He does not inherently change over time. Further, the specific knowledge that the original argument claims He does not have... does not (and cannot) even exist to be had.
In conclusion, God does not change over time, He experiences no tensed facts personally, but He is aware of the truth value of tensed facts as they apply to us, and, clearly, none of this infringes upon His omniscience.