"Un an" is a unit of time. It is always used for age. It is used for ordinal numbers (1, 2, 3, etc) "Un an en France." - One year in France - being a point in time, that it is the point in time that you have been in France for one year.

"Une année" is used when describing not the unit, but the full span of time. For example "l'année derière"-last year. ; "quelques année" - a few years ; "Je suis en France pendant une année." I am in France for a year - for the period, duration, of one year.

For years (or months, days, hours, etc., or centuries, etc.), « dans x years » always means “in x years”, starting from the timeline of the speaker.

« en » is used when you want to point at a specific year or month (that's all): « en 2014 » (so, « dans 1 an » from now), « en mai » (so, « dans 2 mois » from now).

Here is the equivalent of « en » for (almost?) everything else : for eras, it's « à l' » (« à l'ère tertiaire »); for millenniums and centuries, it's « au » (« au deuxième millénaire », « au quinzième siècle ») and we use ordinals; for days, it's « le » (« le 24 mai », « le mardi »); and for hours it's « à » (« à 17 heure »).

in theory your question is legitimate. En un an should have been accepted there. It all depends on the context. In a year i will be in Madrid : dans un an je serai a Madrid How many times have you visited me in a year? = combien de fois m'as tu rendu visite en un an There you can see the difference in usage. I hope this helps.

Pendant means during. I think of it as pending, as in while something else was pending, but that's just a memory thing I made up.

Google translate is often wrong. Also check out www.reverso.net. It is more often right, compared to google translate - but also it is not foolproof. Language is such a complex thing.