I will call you within a week.
““Woche” (week) is a feminine noun, so “einer” is the dative form of the indefinite article “eine,” used here because “innerhalb” requires the genitive case, but in spoken German, dative is often used instead.”
I didn’t get it. After all, “einer” comes from dative or genitive case?
Yes, in this example it actually doesn’t matter if you choose genitive or dative, because it will be “einer Woche” either way. If you switch the noun, the difference becomes apparent:
within one week = innerhalb einer Woche
within one month = innerhalb eines Monats / einem Monat
Great! Thank you!
P.S: So, after all, you didn’t left Clozemaster, as you was thinking about in your famous YTB video, right, Sir? 
I still do the occasional drive-by in these forums just to see what I’m missing