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WeekOf

Function WeekOf( const AValue : TDateTime) : Word

Example​

procedure ScriptEvent(var Value: variant);
begin
  Value := WeekOf(EncodeDate(2024, 12, 31));
  // Value is 1 because this Tuesday belongs to ISO week 1 of 2025.
end;

Usage​

Returns the ISO 8601 week-of-year number as an exact alias of WeekOfTheYear.

Parameters

NameTypeDescription
AValueTDateTimeA valid encoded date/time. Its time-of-day component does not affect the result.

Returns

The one-based ISO week number, from 1 through 53.

Behaviour

The date is decoded without changing it or consulting the host clock, locale or timezone. Early January dates can return week 52 or 53 of the preceding ISO week-year; late December dates can return week 1 of the following ISO week-year.

Errors

Velox adds no exception handler. Date conversion or range errors raised by the installed RTL propagate to the script.

Additional Technical Info​

Returns the ISO 8601 week-of-year number containing AValue. The result is from 1 through 53. WeekOf is an exact alias of WeekOfTheYear; it does not return the associated ISO week-year.

Implementation

Velox binds this declaration directly to the installed System.DateUtils.WeekOf. That function calls the one-argument WeekOfTheYear, which delegates to DecodeDateWeek. The decoder uses Monday through Sunday weeks and assigns a week to the year containing at least four of its days. Internally, the first Thursday rule is implemented by shifting the day-of-year calculation according to the weekday of 1 January.

Edge cases and quirks

  • The function name does not imply a simple seven-day block counted from 1 January. ISO year boundaries govern the result.
  • Velox exposes only the week number. Use DecodeDateWeek when the corresponding ISO week-year and weekday are also required.
  • The routine expects an encodable Gregorian TDateTime. A value below the supported year-1 boundary reaches an invalid year during the recursive decode and the installed RTL raises a conversion error.
  • Free Pascal exposes the same named routine, but its documentation is compatibility guidance; the installed Delphi 37.0 source is authoritative for Velox.

Performance and concurrency

The calculation is fixed-size date arithmetic, allocates no objects and uses no Velox shared state.

Related entries

  • WeekOfTheYear — returns the same week number through the underlying named function.
  • DecodeDateWeek — also returns the ISO week-year and ISO weekday.
  • WeekOfTheMonth — applies the related four-day ownership rule at month boundaries.

External references

Created 2026-07-15