The indefinite article "
ein" has to be declined in congruency with the noun "
Zeit". To decline "
ein" correctly, you need to determine:
- the gender
- the case
of the noun. The grammatical gender of "
Zeit" is female. And this fact rules out "
einen" as option; see this overview regarding how to decline "
ein". So, "
einen" can't be correct in connection with "
Zeit", independent of the case.
So, you finally have to select between "
eine" and "
einer". Here, you'll have to look at the case of the noun (see splattne's answer), so "
einer" is correct.
To pick up your comment regarding "
lang" is quite difficult to handle: just compare the declination table for "
ein" cited above to that overview of the different forms of "
In this case, you need the first three tables for the "positive" (neither comparative, nor superlative) form of "
lang". Here, we need the third table, because there's "
ein", so it's a case of mixed inflection. The noun is singular female dative, so the correct version is "