EN v DANS

Using en as a temporal preposition means "whithin a certain timeframe", which can also be translated as "in" (although "within" is usually better), just as dans. However, they are not interchangeable.

Nous mangeons dans une heure. -- We're eating in one hour (if it is 11 am now, we will eat at noon).
Nous mangeons en une heure. -- We are eating within one hour (no matter what time it is when we start eating, we'll be finished within one hour).

It is, but this is not exclusive to “dans”, and it is used in the way Chris notes.