Why, it's the Anglo-Saxon period of course! 410 - 1066 When England was a big free for all between the Danes, the Norwegians, the Welsh, the Irish, the Scots and the English themselves ..or the Mercians, the Wessex and the Northumbrians.
Eft he axode, hu ðære ðeode nama wære þe hi of comon. Him wæs geandwyrd, þæt hi Angle genemnode wæron. Þa cwæð he, "Rihtlice hi sind Angle gehatene, for ðan ðe hi engla wlite habbað, and swilcum gedafenað þæt hi on heofonum engla geferan beon."
I am guessing you would write "Det er en historisk periode", and in this case (I mean the en/et case, contrary to the det/den case for instance) the "en" adjectives get no ending and the "et" ones get a t. Or at least I managed to be okay using this rule so far.
Nothing to do with it but I have to write it somewhere. Today Duo wrote to me "imposta il stato". If you know Italian even at a basic level you can see the mistake! I wonder: I am learning languages from people who do NOT know languages????