Date Module

Functions for working with dates (without time) backed by chrono.

The Date module provides a Date type for representing calendar dates. Unlike DateTime which uses Maybe for fallible operations, Date uses Result with typed DateError variants for precise error handling.

Common patterns

import Date

-- Create from components
Date.fromYmd 2024 6 15

-- Parse and manipulate
Date.parseIso "2024-01-15"
    |> Result.map (Date.addDays 30)

Return types

Functions that can fail return Result Date DateError.

Functions

Creation

`Date.today`

() -> Date

Get the current local date

Example:

import Date

Date.today ()

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`Date.fromYmd`

Int -> Int -> Int -> Result Date DateError

Create a Date from year, month, day

Example:

import Date

Date.fromYmd 2024 6 15

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`Date.fromOrdinal`

Int -> Int -> Result Date DateError

Create a Date from year and day of year (1-366)

Example:

import Date

Date.fromOrdinal 2024 100

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Parsing

`Date.parse`

String -> String -> Result Date DateError

Parse a date string using a strftime format

Example:

import Date

Date.parse "15/06/2024" "%d/%m/%Y"

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`Date.parseIso`

String -> Result Date DateError

Parse an ISO 8601 date string (YYYY-MM-DD)

Example:

import Date

Date.parseIso "2024-06-15"

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Components

`Date.year`

Date -> Int

Get the year component

Example:

import Date

case Date.fromYmd 2024 6 15 of
    Ok d -> Date.year d
    Err _ -> 0

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`Date.month`

Date -> Int

Get the month component (1-12)

Example:

import Date

case Date.fromYmd 2024 6 15 of
    Ok d -> Date.month d
    Err _ -> 0

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`Date.day`

Date -> Int

Get the day component (1-31)

Example:

import Date

case Date.fromYmd 2024 6 15 of
    Ok d -> Date.day d
    Err _ -> 0

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`Date.weekday`

Date -> Int

Get the weekday (1=Monday, 7=Sunday)

Example:

import Date

case Date.fromYmd 2024 1 1 of
    Ok d -> Date.weekday d  -- Monday = 1
    Err _ -> 0

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`Date.dayOfYear`

Date -> Int

Get the day of year (1-366)

Example:

import Date

case Date.fromYmd 2024 4 9 of
    Ok d -> Date.dayOfYear d  -- 100
    Err _ -> 0

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`Date.weekNumber`

Date -> Int

Get the ISO week number (1-53)

Example:

import Date

case Date.fromYmd 2024 1 1 of
    Ok d -> Date.weekNumber d
    Err _ -> 0

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`Date.isLeapYear`

Date -> Bool

Check if the year is a leap year

Example:

import Date

case Date.fromYmd 2024 1 1 of
    Ok d -> Date.isLeapYear d  -- True
    Err _ -> False

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`Date.daysInMonth`

Date -> Int

Get the number of days in the month

Example:

import Date

case Date.fromYmd 2024 2 1 of
    Ok d -> Date.daysInMonth d  -- 29 (leap year)
    Err _ -> 0

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Manipulation

`Date.addDays`

Int -> Date -> Date

Add days to a Date

Example:

import Date

case Date.fromYmd 2024 1 1 of
    Ok d -> Date.toIsoString (Date.addDays 10 d)
    Err _ -> "error"

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`Date.addWeeks`

Int -> Date -> Date

Add weeks to a Date

Example:

import Date

case Date.fromYmd 2024 1 1 of
    Ok d -> Date.toIsoString (Date.addWeeks 2 d)
    Err _ -> "error"

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`Date.addMonths`

Int -> Date -> Date

Add months to a Date (handles month-end overflow)

Example:

import Date

case Date.fromYmd 2024 1 31 of
    Ok d -> Date.toIsoString (Date.addMonths 1 d)  -- Feb 29
    Err _ -> "error"

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`Date.addYears`

Int -> Date -> Date

Add years to a Date (handles leap year Feb 29)

Example:

import Date

case Date.fromYmd 2024 2 29 of
    Ok d -> Date.toIsoString (Date.addYears 1 d)  -- Feb 28
    Err _ -> "error"

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Difference

`Date.diffDays`

Date -> Date -> Int

Calculate the difference in days between two dates

Example:

import Date

case (Date.fromYmd 2024 1 1, Date.fromYmd 2024 1 11) of
    (Ok d1, Ok d2) -> Date.diffDays d1 d2  -- 10
    _ -> 0

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Formatting

`Date.format`

String -> Date -> String

Format a date using a strftime format string

Example:

import Date

case Date.fromYmd 2024 6 15 of
    Ok d -> Date.format "%d/%m/%Y" d
    Err _ -> "error"

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`Date.toIsoString`

Date -> String

Format as ISO 8601 (YYYY-MM-DD)

Example:

import Date

case Date.fromYmd 2024 6 15 of
    Ok d -> Date.toIsoString d  -- "2024-06-15"
    Err _ -> "error"

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Comparison

`Date.isBefore`

Date -> Date -> Bool

Check if the first date is before the second

Example:

import Date

case (Date.fromYmd 2024 1 1, Date.fromYmd 2024 12 31) of
    (Ok d1, Ok d2) -> Date.isBefore d1 d2  -- True
    _ -> False

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`Date.isAfter`

Date -> Date -> Bool

Check if the first date is after the second

Example:

import Date

case (Date.fromYmd 2024 12 31, Date.fromYmd 2024 1 1) of
    (Ok d1, Ok d2) -> Date.isAfter d1 d2  -- True
    _ -> False

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`Date.isEqual`

Date -> Date -> Bool

Check if two dates are equal

Example:

import Date

case (Date.fromYmd 2024 6 15, Date.fromYmd 2024 6 15) of
    (Ok d1, Ok d2) -> Date.isEqual d1 d2  -- True
    _ -> False

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`Date.compare`

Date -> Date -> Int

Compare two dates (-1 if before, 0 if equal, 1 if after)

Example:

import Date

case (Date.fromYmd 2024 1 1, Date.fromYmd 2024 12 31) of
    (Ok d1, Ok d2) -> Date.compare d1 d2  -- -1
    _ -> 0

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