Unix timestamps are a duration¹; they're not in any particular timezone. They're "the number of seconds since midnight 1 Jan 1970, UTC" — but while I've used "UTC" there, you can also equally define it as the number of seconds since 19:00 on 31 Dec 1969 in America/New_York. It's the epoch itself that requires a timezone, not the duration since the epoch.

¹iiiish. Let's forget leap seconds exist.