[ecfisa]

Hola.

Un modo puede ser,

Código Delphi [-]
function AmPmTo24(const ampm: TTime): string;
var
  F: TFormatSettings;
begin
  F := FormatSettings;
  try
    with FormatSettings do // para XE3 o sup.
    begin
      LongTimeFormat  := 'hh:mm:ss';
      TimeSeparator   := ':';
      Result := TimeToStr(ampm);
    end;
  finally
    FormatSettings := F;
  end;
end;

Ejemplo de uso:

Código Delphi [-]
procedure TForm1.Button1Click(Sender: TObject);
begin
  ShowMessageFmt('%s%s%s',[AmPmTo24(StrToTime('11:59:59 AM')), #10,
    AmPmTo24(StrToTime('11:59:59 PM'))]);
end;

Saludos