I should put this somewhere

default.nix

{ haskellPackages }:
  haskellPackages.callCabal2nix "in-time-zone" ./. {}

in-time-zone.cabal

cabal-version:       >=1.10
name:                in-time-zone
version:             0.1.0.0
author:              Brad Parker
maintainer:          hi@bradparker.com
build-type:          Simple

executable in-time-zone
  main-is:             Main.hs
  build-depends:       base >=4.14 && <4.15,
                       time
  default-language:    Haskell2010

Main.hs

module Main where

import Control.Applicative ((<|>))
import Data.Time (TimeZone, ZonedTime, utc, utcToZonedTime, zonedTimeToUTC)
import Data.Time.Format.ISO8601 (iso8601ParseM)
import System.Environment (getArgs)

inTimeZone :: TimeZone -> ZonedTime -> ZonedTime
inTimeZone timezone = utcToZonedTime timezone . zonedTimeToUTC

main :: IO ()
main = do
  [zoneString, timeString] <- getArgs
  print =<< inTimeZone
    <$> iso8601ParseM zoneString
    <*> (iso8601ParseM timeString <|> utcToZonedTime utc <$> iso8601ParseM timeString)

Setup.hs

import Distribution.Simple
main = defaultMain