That's the time span during which Beowulf was set down in ink,
along with much of the rest of the OE corpus of literature.
It took a couple of generations after the Norman conquest for
the French elements to creep in and alter the language to the point of calling
it Middle English. (The language of Chaucer.)
Dæg
i know that around 700 or 800 old english was no more it was then middle
english