Comparing two dates

Elm:
Comparing two dates

How to:

Elm makes date comparisons straightforward. Let’s say you’ve got two dates. Here’s how you’d check which one comes first:

import Time exposing (Posix)
import Date

compareDates : Posix -> Posix -> Order
compareDates date1 date2 =
    if date1 < date2 then
        LT  -- date1 is earlier than date2
    else if date1 > date2 then
        GT  -- date1 is later than date2
    else
        EQ  -- dates are the same

-- Sample Usage:
let
    date1 = Date.fromPosix <| Time.millisToPosix 1650931200000 -- Add your first date in POSIX time
    date2 = Date.fromPosix <| Time.millisToPosix 1651017600000 -- And your second date in POSIX time
in
compareDates date1 date2
-- Output will be either LT, GT, or EQ

You can also calculate the difference in milliseconds:

timeDifference : Posix -> Posix -> Time.Duration
timeDifference date1 date2 =
    Time.millisToPosix date1 - Time.millisToPosix date2

-- Sample Usage:
let
    date1 = Date.fromPosix <| Time.millisToPosix 1650931200000
    date2 = Date.fromPosix <| Time.millisToPosix 1651017600000
in
timeDifference date1 date2
-- Output: Duration in milliseconds

Deep Dive

Elm stores dates as Posix, representing milliseconds since Unix epoch (1 January 1970, UTC). This is a common approach, sharing its roots with Unix Time, and it eases date manipulation and storage.

While Elm’s core library provides basic date handling, some alternatives like justinmimbs/date exist for more complex operations.

When implementing date comparisons, remember time zones can complicate things. Elm’s Time module assumes UTC, which means you’re spared daylight saving headaches, but you might need to adjust for local time zones in your application.

Elm:Comparing two dates

How to:

Deep Dive

See Also

Elm:
Comparing two dates