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0024

Week 53 of Last Year

Basics
Easy
dates
iso-week
leap-year

Week 53 of Last Year

The weekly sales summary groups posted invoices under an ISO week key such as 2026-W01, and finance wants the month-end, quarter and leap-year helpers to live next to it. The first version keyed every invoice on its calendar year — and every January the summary showed invoices from New Year's Day under "week 53 of 2021", a week that does not exist: 1 January 2021 belongs to week 53 of 2020. AL's date toolbox already knows the ISO rules; this task is about calling the right function and reading its result correctly.

Requirements

Create a codeunit named "Period Helper" with six public procedures:

procedure IsoWeekKey(TheDate: Date): Text
procedure WeekMonday(TheDate: Date): Date
procedure EndOfMonth(TheDate: Date): Date
procedure DaysInMonth(TheDate: Date): Integer
procedure IsLeapYear(Year: Integer): Boolean
procedure QuarterOf(TheDate: Date): Integer

The toolbox: AL takes a date apart in two ways. Date2DMY returns the calendar day, month and year. Date2DWY returns the day of the week (1 = Monday … 7 = Sunday), the ISO week number, and — the part this task depends on — the ISO week-numbering year: 2020 for 1 January 2021, 2026 for 29 December 2025. DMY2Date and DWY2Date build a date back from those pieces, and DWY2Date(1, 1, 2026) correctly returns 29 December 2025 — the Monday that starts week 1 of 2026. CalcDate moves a date by a date formula, and the formula <CM> (current month) lands on the last day of the reference date's month. A dead end to know about: Date2DMY(TheDate, 3) and TheDate.Year() return the calendar year — right for fifty weeks a year and wrong on exactly the dates the tests probe.

Rules:

  1. IsoWeekKey returns the ISO 8601 week key: the four-digit ISO week-numbering year, a hyphen, a capital W, and the week number zero-padded to two digits — 2024-W24, 2026-W08. ISO weeks run Monday to Sunday and week 1 is the week containing 4 January, so the first days of January can belong to week 52 or 53 of the previous year and the last days of December can belong to week 1 of the next: 1 January 2021 is 2020-W53, 29 December 2025 is 2026-W01.
  2. WeekMonday returns the Monday that starts the date's week: the date itself when it is a Monday, otherwise the nearest Monday before it — six days back for a Sunday. The Monday may lie in the previous month or year.
  3. EndOfMonth returns the last day of the date's month — 29 February in a leap year, 28 otherwise, 31 December for a December date. The last day of a month maps to itself.
  4. DaysInMonth returns the number of days in the date's month, 28 to 31.
  5. IsLeapYear follows the Gregorian rules: a year divisible by 4 is a leap year, except century years, which are leap years only when divisible by 400 — 2000 was one, 1900 and 2100 are not.
  6. QuarterOf returns the calendar quarter 1 to 4: January–March is 1, April–June is 2, July–September is 3, October–December is 4.

You may rely on this: every date the tests pass lies between 1900 and 2100, and every year passed to IsLeapYear lies between 1800 and 2400.

Pick your codeunit's object ID in the 50100–50199 range and reference other objects by name, never by ID.

What the tests check

The tests call each procedure with fixed dates: a mid-year date (2024-W24), 1 January 2021 (2020-W53), 29 December 2025 (2026-W01) and 18 February 2026 (2026-W08, note the zero) for the week key; a Sunday whose Monday is six days back, a Monday that maps to itself, 1 January 2021 (Monday 28 December 2020) and 31 December 2025 (Monday 29 December 2025) for the week Monday; February 2024, February 2023, a December date and a date that already is the last day of its month for the month end; February 2024 and February 2100 for the month length; 2024, 2023, 1900 and 2000 for the leap flag; and 31 March, 1 April and a December date for the quarter. Every procedure is also graded on a generated input compared against an independent calendar computation — the ISO year taken from the week's Thursday, a weekday count back to Monday, a day-by-day walk to the month end, February's actual length — so hardcoding the examples fails. Week-key comparisons are exact text: four-digit year, hyphen, capital W, two digits.

Learn More

Hint 1
Every helper is one or two calls into the platform's date toolbox — no loops, no table of month lengths. Date2DWY takes a date apart by week (day of week, week number, and the year that week belongs to), Date2DMY takes it apart by calendar month, DWY2Date and DMY2Date put the pieces back together, and CalcDate moves a date by a formula.
Hint 2
The third value Date2DWY returns is the ISO week-numbering year — 2020 for 1 January 2021, 2026 for 29 December 2025 — exactly what the week key needs; Date2DMY's third value and Date.Year() give the calendar year, which is wrong on those days. Pass that same week-year, not the calendar year, together with weekday 1 and the week number to DWY2Date when you rebuild the Monday: DWY2Date(1, 1, 2026) really is 29 December 2025.
Hint 3
Zero-padding: Format with the format string '<Integer,2><Filler Character,0>' renders 7 as 07 — or prefix a '0' yourself when the week is below 10. The date formula <CM> in CalcDate jumps to the last day of the reference date's month, leap February included; the day number of that date is the month's length, and February's length tells you whether a year is leap — build 1 February of that year with DMY2Date and ask. The quarter is integer arithmetic on the month number: months 1–3 give 1, 4–6 give 2, and so on.
ALBusiness Central 28.5
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