Delphi中本年、本月、本周的第一天和最后天

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