try this:

========zero cumulative error timing routine, from Black Roman============
//initialize the timer
void t0_init(unsigned short duration) {
  TH0=duration >> 8;  //th0 has the msb
  TL0=duration & 0xff; //tl0 has the lsb
}

/********定时器T0中断函数**********/  
void t0(void) interrupt 1   
{  
static unsigned long total_ticks=0;  //initialize total_ticks
//clear the flag
//TH0=(65536-50000)/256;  
//TL0=(65536-50000)%256;//这样的定时数据，是不是正好是50000*1微秒=50毫秒？  
  total_ticks += 65536ul;  //advance duration;
  if (total_ticks >= DURATION) { //desired duration has reached;
    total_ticks -= DURATION;  //update total_ticks but retain the error
    t0_flag=1;  //set the flag
  }
}

==========================

DURATION is a macro or global variable that determines how long t0_flag will be set.
t0_flag is set when the desired ticks, as specified by DURATION, have been reached.

so if each tick is 1us, and you want t0_flag is set every 500ms, you set DURATION to 500000ul.

the accuracy here depends entirely on the oscillator, and the approach is highly portable.