#include <DateUtils.hpp>

TDateTime pressTime;

void __fastcall TForm1::ButtonMouseDown(TObject *Sender, TMouseButton Button, TShiftState Shift, float X, float Y)
{
  pressTime = Now();
}

void __fastcall TForm1::ButtonMouseUp(TObject *Sender, TMouseButton Button, TShiftState Shift, float X, float Y)
{
  __int64 duration = MillisecondsBetween(Now(), pressTime);
  ShowMessage("Кнопка была нажата " + IntToStr(duration) + " миллисекунд");
}