Undeclared identifier

unit MainForm;interfaceuses
   Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
   Dialogs, untClassStudent, untClassTeacher, StdCtrls;type
   TForm1 = class(TForm)
   lbTeacher: TListBox;
   lbStudent: TListBox;
   btnTeacher: TButton;
   btnStudent: TButton;
   procedure btnTeacherClick(Sender: TObject);
   procedure btnStudentClick(Sender: TObject);
   private
   { Private declarations }
   public
   { Public declarations }
   end;var
   Form1: TForm1;
   ATeacher:TTeacher;
   AStudent:TStudent;implementation
{$R *.dfm}procedure TForm1.btnTeacherClick(Sender: TObject);
begin
   ATeacher:=TTeacher.Create('马萍',Female,28,'讲师');
   lbTeacher.Items.Add('教师姓名：'+ATeacher.Name); //显示ATeacher变量的Name域
   lbTeacher.Items.Add('教师性别：'+ATeacher.Sex);
   lbTeacher.Items.Add('教师年龄：'+IntToStr(ATeacher.Age));
   lbTeacher.Items.Add('教师职称：'+ATeacher.TeachPost);
   lbTeacher.Items.Add(ATeacher.Talk);
   lbTeacher.Items.Add('类变量所分配的存储字节数：'+IntToStr(SizeOf(ATeacher)));
   lbTeacher.Items.Add('类实例所分配的存储字节数：'+IntToStr(TTeacher.InstanceSize));
   ATeacher.Free; //释放对象实例
   Ateacher:=nil;
end;procedure TForm1.btnStudentClick(Sender: TObject);
begin
   AStudent:=TStudent.Create; //创建对象实例
   AStudent.Name:='李大江';
   AStudent.Sex:=True;
   AStudent.Birthday:=EncodeDate(1986,2,18);
   lbStudent.Items.Add('学生姓名：'+AStudent.Name); //显示ATeacher变量的Name域
   if AStudent.Sex then
   lbStudent.Items.Add('男')
   else
   lbStudent.Items.Add('女');
   lbStudent.Items.Add('学生生日：'+DateToStr(AStudent.Birthday));
   lbStudent.Items.Add(AStudent.Talk);
   lbStudent.Items.Add('类变量所分配的存储字节数：'+IntToStr(SizeOf(AStudent)));
   lbStudent.Items.Add('类实例所分配的存储字节数：'+IntToStr(TStudent.InstanceSize));
   AStudent.Free; //释放对象实例
end;end.unit untClassTeacher;interfaceuses
   controls, Dialogs, DateUtils;type
   TTeacher=class
   private
   FName:string; //姓名
   FSex:Boolean; //性别
   FBirthday:TDate; //出生日期
   FTeachPost:string;
   function GetAge: Integer;
   function GetSex: string;
   procedure SetAge(const Value: Integer);
   procedure SetSex(const Value: string); //职称
   public
   constructor Create(NValue:string;SValue:Boolean;BValue:TDate;TPValue:string);overload;//构造创建方法（重载）
   constructor Create(N,S:string;A:Integer;T:string);overload;
   destructor Destroy;override; //析构方法，销毁对象（覆盖）
   function Talk:string;
   property Name:string read FName write FName;
   property Sex:string read GetSex write SetSex;
   property Age:Integer read GetAge write SetAge;
   property TeachPost:string read FTeachPost write FTeachPost;
end;implementation{ TTeacher }constructor TTeacher.Create(NValue:string;SValue:Boolean;BValue:TDate;TPValue:string); //TTeacher.一定要全称！！切记！
begin
   FName:=NValue;
   FSex:=SValue;
   FBirthday:=BValue;
   FTeachPost:=TPValue;
end;function TTeacher.Talk:string;
begin
   Result:='I am a teacher';
end;destructor TTeacher.Destroy ;
begin
   ShowMessage('Instance object has been destroyed.');
end;constructor TTeacher.Create(N, S: string; A: Integer; T: string);
begin
   Name:=N;
   Sex:=S;
   Age:=A;
   TeachPost:=T;
end;function TTeacher.GetAge: Integer;
begin
   Result:=YearOf(Now)-YearOf(FBirthDay);
end;function TTeacher.GetSex: string;
begin
   if FSex then
   Result:='男'
   else
   Result:='女';
end;procedure TTeacher.SetAge(const Value: Integer);
begin
   if(Value<=200) and (Value>0) then
   FBirthDay:=EncodeDate(YearOf(Now)-Value,1,1)
   else
   raise Exception.Create('年龄输入有误');
end;procedure TTeacher.SetSex(const Value: string);
begin
   if(Value='男') or (UpperCase(Value)='MALE') or (UpperCase(Value)='TRUE') then
   FSex:=True
   else if(Value='女') or (UpperCase(Value)='FEMALE') or (UpperCase(Value)='FALSE') then
   FSex:=False
   else
   raise Exception.Create('性别输入有误');
end;end.
unit untClassStudent;interfaceuses
   controls;type
   TStudent=class
   Name:string; //姓名
   Sex:Boolean; //性别
   Birthday:TDate; //出生日期
   function Talk:string;
end;implementation{ TStudent }function TStudent.Talk:string;
begin
   Result:='I am learning programing.';
end;end.
运行出现的问题是：
[Error] untClassTeacher.pas(62): Undeclared identifier: 'YearOf'
[Error] untClassTeacher.pas(76): Undeclared identifier: 'EncodeDate'
[Error] untClassTeacher.pas(76): Undeclared identifier: 'YearOf'
[Error] untClassTeacher.pas(78): Undeclared identifier: 'Exception'
[Error] untClassTeacher.pas(78): There is no overloaded version of 'Create' that can be called with these arguments
[Error] untClassTeacher.pas(83): Undeclared identifier: 'UpperCase'
[Error] untClassTeacher.pas(83): Operator not applicable to this operand type
[Warning] untClassTeacher.pas(83): Combining signed and unsigned types - widened both operands
[Error] untClassTeacher.pas(85): Operator not applicable to this operand type
[Warning] untClassTeacher.pas(85): Combining signed and unsigned types - widened both operands
[Error] untClassTeacher.pas(88): Undeclared identifier: 'Exception'
[Error] untClassTeacher.pas(88): There is no overloaded version of 'Create' that can be called with these arguments
[Fatal Error] MainForm.pas(7): Could not compile used unit 'untClassTeacher.pas'