The full poem survives in the manuscript known as the Nowell Codex.
It is, because of the fire in 1731, many of the manuscripts were lost.
The importance of dating such literature is not only historical, for, in interpreting literature, knowledge of the setting in which a work was composed is of great importance.
Mrs X fired her last nanny because she was dating and that gives Annie problems when Harvard Hottie who lives in the same building asks her out on a date.
When Annie takes Grayer to the play date at the house where he and another boy are decorating a cake, a glob of the pink icing lands on the front of Annie's white t-shirt.
Old English literature is largely preserved in manuscripts of the late tenth, eleventh and twelfth centuries, and Beowulf is no exception, surviving in a late 10th- or early 11th-c. Based on external evidence such as historical references or authorship, some poetry, like Cædmon's Hymn, can be dated as early as the 7th-c., whilst other poems, like The Death of Edgar, can be dated as late as the mid-eleventh century.
But the main body of Old English literature, including Beowulf, cannot be so easily dated, except roughly to sometime during a period of two or three hundred years or more.
Whether by evil intent or not, the result was a profound tectonic shift in the understanding of the poem, which through these machinations was handed over to a long series of literary critics of all ilks and genders.