Cookbooks
Elm
Chapter 7. Dates and Times
Calculating a date in the future or past

Elm:
Calculating a date in the future or past

How to:

Elm’s Time module and the justinmimbs/time-extra package let us mess with dates easily.

import Time exposing (Posix)
import Time.Extra as TimeExtra

--calculateDate : Int -> Posix -> Posix
-- @deltaDays: number of days to add (negative to subtract)
-- @fromDate: starting date in Posix format

calculateDate deltaDays fromDate =
    TimeExtra.add TimeExtra.days deltaDays fromDate

-- Usage
-- Don't forget, Elm counts time in milliseconds since Unix epoch.

sampleDate = Time.millisToPosix 1580515200000  -- February 1, 2020 00:00:00 UTC
futureDate = calculateDate 10 sampleDate       -- Adds 10 days
pastDate = calculateDate -15 sampleDate        -- Subtracts 15 days

-- sample outputs:
-- futureDate -> 1581552000000  -- February 12, 2020 00:00:00 UTC
-- pastDate -> 1580006400000    -- January 17, 2020 00:00:00 UTC

Deep Dive

Back in the day, dealing with dates in programming was a pain. Different systems, formats, and time zones gave everyone a headache. Elm’s Time module, based on the Unix Time system (milliseconds since 1970), standardizes this. The justinmimbs/time-extra package further simplifies handling operations on dates, like adding or subtracting days.

Alternatives? Other languages have their own libraries, like Python’s datetime or JavaScript’s Date. But Elm’s approach offers strong typing and purity, reducing bugs.

Beyond adding days, you can also work with months, years, or even hours and minutes. The functions in Elm and in packages like time-extra focus on immutability and pure functions—this means no side effects. When you calculate a new date, the original stays unchanged.

See Also

Comparing two dates