Link - Zman Maariv

There is a major discussion when the earliest time to daven maariv is in New York. In Israel, it is genrally accepted that 20 minutes after shkiya one may daven.

Rav Moshe Feinstein held [if I am not mistaken] that in New York one must wait 45 minutes until maariv. The Lubavitcher Rebbe held 36 minutes [even chasidim believe in zmanei tfilla:-)].

[In galus, ruchniyus goes slower. Gotta wait longer to daven....]

For an explanation of the former opinion see the Igros Moshe. For an explanation of the latter opinion see here.