void Timerinterrupt(void)
{
TR0 = 0 //stop timer
TH0 = <>
TL0 = <>
TR0 = 1; //start running timer
z++;
if(z == 20)
{
z = 0;
seconds++;
}
}

If TL0 does not require an update may not have to update the timer. If you give the timer highest priority that will help. Disabling the interupts will hurt. An autoreload timer will be the best. but note that most mpu/Xtals are not good enough to make a clock. That requires much more accuracy.