It's pointless to try and draw parralels from a world in which the concepts in game were inspired from?
The Nords clearly aren't inspired by some elements of real life Scandinavian culture, they're just... Nords... Even their names weren't drawn from concepts in our world.
Eras and dating are something that definitely aren't in our world... I mean, it's not like it's CE 2012 or anything.
Literature and science..? Real life doesn't have books or science!
But to stay on topic: OP, I would estimate somewhere at about CE 500 - CE 1000 as stated above. The art style is the closest to what we'd know as the Middle Ages, here on Earth.
Of course it is, when talking about a fantasy universe that combines so many influences from so many different time periods and cultures.
Its like looking at a bowl of alphabet spaghetti and asking what letter it is.
Many people here say something incredibly silly like 1000 AD or something. All fine and dandy, but they use a type of glass not invented since the late 1600's. The printing of books is ubiquitous and not just restricted to religious texts, again, late renaissance.
Then there is the space travel and the construction of pocket dimensions.
Good luck putting that in medieval times.