void Time4Init(void)
{
    TIM_TimeBaseInitTypeDef  TIM_TimeBaseStructure;
    RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM4, ENABLE);
    TIM_TimeBaseStructure.TIM_Period = 65535;
    TIM_TimeBaseStructure.TIM_Prescaler = (72 - 1);//72M / 72 = 1us
    TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;
    TIM_TimeBaseInit(TIM4, &TIM_TimeBaseStructure);
}

从0开始开启定时器TIM4  TIM_GetCounter(TIM4)得到的计数值 如何计算时间  单位是多少 拜托讲下