Given an IANA Timezone and a local time, parse it into a Maybe Posix?

Question

Is there a way in Elm to take a local time (such as the string 2019-03-18T09:10:12.4; no offset specifed) and a timezone (such as Australia/Sydney) to a possible Posix value (i.e, that time converted to UTC), without using ports?

---

There's waratuman/time-extra, but it seems to only work on the Date portion. And sadly rtfeldman/elm-iso8601-date-strings doesn't take timezones.

In JS, there's options such as moment-tz and date-fns-timezone, but it would be much simpler to avoid JS interop for frequent date parsing.

Answer 1

If we can assume that all of your dates come without a timezone, and that you always want to cast them to Australia/Sydney before converting them to Posix, you should be able to just concatenate the offset yourself to create 2019-03-18T09:10:12.4+11:00:

toPosix : String -> String -> Maybe Posix
toPosix timezone timestampWithoutTimezone =
    let
        timestampWithTimezone = timestampWithoutTimezone ++ timezone
    in
        timestampWithTimezone |> toTime |> Result.toMaybe


{- If you always want to use Australia/Sydney -}

localPosix =
    toPosix "+11:00"

posix =
    localPosix "2019-03-18T09:10:12.4"

{- Custom timezone -}

posix =
    toPosix "+11:00" "2019-03-18T09:10:12.4"