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Daydate Library

Date arithmetic and temporal logic for legal deadlines and time-based rules. Follows ISO 8601 conventions. Can be imported into L4 files with IMPORT daydate. Automatically imports date-compat for backwards-compatible DATE accessors.

Location

jl4-core/libraries/daydate.l4

Features

  • Date construction from day/month/year, serial numbers, strings, or DATETIME extraction
  • Date arithmetic (add/subtract days, weeks, months, years)
  • Weekday and weekend detection
  • Week-of-year calculations
  • Month and year helpers
  • Leap year detection
  • String parsing for JSON interoperability
  • DATETIME date extraction

Key Functions

Constants:

  • Monday through Sunday (0-6)
  • January through December (1-12)
  • Days in a week, Days in a year, etc.

Date Construction:

  • Date day month year - Create date from components
  • Date serialNumber - Create date from serial number
  • Date str - Parse date string (ISO format, ignores time/timezone if present), returns MAYBE DATE
  • Date dt - Extract date from DATETIME (ignores time and timezone)
  • Week weekNumber year - First day of week
  • Month month year - First day of month
  • Year year - First day of year

Date Arithmetic:

  • date PLUS days - Add days to date
  • date MINUS days - Subtract days from date
  • the day after, the week before, etc.

Queries:

  • Weekday of date - Get weekday (0-6)
  • Week of the year date - Get ISO week number
  • is weekend, is weekday - Check day type
  • is leap year - Check leap year

Comparators:

  • the earlier of d1 d2
  • the later of d1 d2
  • d1 is before d2, d1 is after d2
  • Standard comparison operators: LESS THAN, GREATER THAN, AT MOST, AT LEAST

Example: Date Calculations

-- Daydate Example: Date calculations and temporal logic
IMPORT daydate

-- Date construction from components
DECIDE `Christmas 2024` IS Date 25 12 2024
DECIDE `New Year 2025` IS Date 1 1 2025

#EVAL `Christmas 2024`
#EVAL `New Year 2025`

-- Date from ISO string (returns MAYBE DATE, ignores time/timezone if present)
DECIDE `from iso date` IS Date "2025-06-15"
DECIDE `from iso datetime` IS Date "2025-06-15T10:30:00Z"
#EVAL `from iso date`       -- JUST OF (DATE OF 15, 6, 2025)
#EVAL `from iso datetime`   -- JUST OF (DATE OF 15, 6, 2025)

-- Date arithmetic
DECIDE `week later` IS `Christmas 2024` PLUS 7
#EVAL `week later`  -- January 1, 2025

DECIDE `week before` IS `Christmas 2024` MINUS 7
#EVAL `week before`  -- December 18, 2024

-- Date components
#EVAL DATE_DAY `Christmas 2024`    -- 25
#EVAL DATE_MONTH `Christmas 2024`  -- 12
#EVAL DATE_YEAR `Christmas 2024`   -- 2024

-- Weekday calculations
DECIDE `day of week` IS `Weekday of` `Christmas 2024`
#EVAL `day of week`  -- Wednesday (3)

#EVAL `Name of weekday` `Christmas 2024`  -- "Wednesday"

-- Weekend checks
#EVAL `is weekend` `Christmas 2024`   -- FALSE
#EVAL `is weekday` `Christmas 2024`   -- TRUE

DECIDE `last Saturday in 2024` IS Date 28 12 2024
#EVAL `is weekend` `last Saturday in 2024` -- TRUE

-- Week of year
#EVAL `Week of the year` `Christmas 2024`  -- 52

-- Leap year
#EVAL `is leap year` 2024  -- TRUE
#EVAL `is leap year` 2023  -- FALSE

-- Relative time phrases
DECIDE `tomorrow` IS `the day after` `Christmas 2024`
#EVAL `tomorrow`

DECIDE `next week` IS `the week after` `Christmas 2024`
#EVAL `next week`


-- Date comparison (via operator overloads)
#EVAL `Christmas 2024` < `New Year 2025`   -- TRUE
#EVAL `Christmas 2024` >= `New Year 2025`  -- FALSE

-- Readable comparisons
DECIDE `christmas is before new year` IS `Christmas 2024` `is before` `New Year 2025`
DECIDE `new year is after christmas` IS `New Year 2025` `is after` `Christmas 2024`
#EVAL `christmas is before new year`  -- TRUE
#EVAL `new year is after christmas`   -- TRUE

-- Month helpers
DECIDE `July 4th` IS July 4 2024
#EVAL `July 4th`
#EVAL `Name of month` `July 4th`  -- "July"

See daydate.l4 source for all functions.