The Astral is not completely timeless, just in regard to very specific processes (namely, all biology). Natural healing, aging, hunger, thirst, tiredness, disease and poison don't progress there, but if you travel on March the 3rd to the Astral and spend a month there, you'll return on April the 3rd. Otherwise the Githyanki would be constantly invading from our perspective, while building up an army every century from their perspective.