これが私の解決策です。もっとコンパクトなものはありますか?
> time_from_client = "2001-03-30T19:00:00-05:00"
=> "2001-03-30T19:00:00-05:00"
> time_from_client.to_datetime
=> Fri, 30 Mar 2001 19:00:00 -0500
> timezone_offset = time_from_client.to_datetime.offset.numerator
=> -5
> tz = ActiveSupport::TimeZone[timezone_offset]
=> (GMT-05:00) America/New_York
> tz.class
=> ActiveSupport::TimeZone