获取当天是星期几，计算两个日期相隔时间

function TFormMain.GetWeek: string;
var
  date:SYSTEMTIME;
begin
  GetLocalTime(date);
  case date.wDayOfWeek of
    0:Result:='星期天';
    1:Result:='星期一';
    2:Result:='星期二';
    3:Result:='星期三';
    4:Result:='星期四';
    5:Result:='星期五';
    6:Result:='星期六';
  end;
end;

 返回值为当天的星期几

function GetSubDateTime(S1, S2:string): string;
var
  T1, T2: TDateTime;
  D, H, M, S: Integer;
  Value: Int64;
begin
  T1 := StrToDateTime(S1);
  T2 := StrToDateTime(S2);
  Value := SecondsBetween(T1, T2);   //两个时间之间相差的秒数
  D := Value div SecsPerDay;
  H := Value mod SecsPerDay div SecsPerHour;
  M := Value mod SecsPerDay mod SecsPerHour div SecsPerMin;
  S := Value mod SecsPerDay mod SecsPerHour mod SecsPerMin;
  result := Format('%2d天 %.2d:%.2d:%.2d',[D, H, M, S]); // 返回几天：几时：几分：几秒
end;