Delphi DateUtils

uses DateUtils;

function StartOfTheYear(const AValue: TDateTime): TDateTime;
function EndOfTheYear(const AValue: TDateTime): TDateTime;
function StartOfAYear(const AYear: Word): TDateTime;
function EndOfAYear(const AYear: Word): TDateTime;

function StartOfTheMonth(const AValue: TDateTime): TDateTime;
function EndOfTheMonth(const AValue: TDateTime): TDateTime;
function StartOfAMonth(const AYear, AMonth: Word): TDateTime;
function EndOfAMonth(const AYear, AMonth: Word): TDateTime;

function StartOfTheWeek(const AValue: TDateTime): TDateTime;            {ISO 8601}
function EndOfTheWeek(const AValue: TDateTime): TDateTime;             {ISO 8601}
function StartOfAWeek(const AYear, AWeekOfYear: Word;  const ADayOfWeek: Word = 1): TDateTime;　　　　{ISO 8601}
function EndOfAWeek(const AYear, AWeekOfYear: Word;  const ADayOfWeek: Word = 7): TDateTime; 　　　　{ISO 8601}