He stated a time period, but not neccisarrily a specific one.
1000 yrs. could easily be stated to mean simply 'a long time'.
Its like the 144,000 listed in Rev. Do you think its EXACTLT 144,000 people, or just a common Hebrew expression for "a lot"?
Same with Jesus telling Peter that we must forgive 7 X 70 times.
It does not mean that the 491st time we do not have to forgive.