Funcion para obtener los minutos entre 2 Horas

[TOPX]

Un programa básico para guiarse:

Label1 tiene la hora actual
Edit1 tiene la hora a restar
Label2 tiene horas, minutos y segundos resultantes

Código Delphi [-]
unit Unit1;

interface

uses
  Winapi.Windows, System.SysUtils, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.StdCtrls, Vcl.ExtCtrls;

type
  TForm1 = class(TForm)
    Edit1: TEdit;
    Button1: TButton;
    Label1: TLabel;
    Label2: TLabel;
    Timer1: TTimer;
    procedure Timer1Timer(Sender: TObject);
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
var
  dif: Extended;
begin
  dif := Time() - StrToTime(Edit1.Text);
  Label2.Caption := FormatDatetime('hh:nn:ss', dif);

  if dif < 0 then
  begin
    Label2.Caption := '- ' + Label2.Caption;
  end;
end;

procedure TForm1.Timer1Timer(Sender: TObject);
begin
  Label1.Caption := TimeToStr(Time()) + ' menos ';
end;

end.

Código Delphi [-]
object Form1: TForm1
  ClientHeight = 243
  ClientWidth = 527
  object Label1: TLabel
    Left = 56
    Top = 88
    Width = 31
    Height = 13
    Caption = 'Label1'
  end
  object Label2: TLabel
    Left = 384
    Top = 88
    Width = 31
    Height = 13
    Caption = 'Label2'
  end
  object Edit1: TEdit
    Left = 144
    Top = 85
    Width = 121
    Height = 21
    TabOrder = 0
    Text = '11:11:10'
  end
  object Button1: TButton
    Left = 288
    Top = 83
    Width = 75
    Height = 25
    Caption = '='
    TabOrder = 1
    OnClick = Button1Click
  end
  object Timer1: TTimer
    OnTimer = Timer1Timer
    Left = 256
    Top = 128
  end
end

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