百思不得其解，请求大神帮忙看看这个数字钟的程序为什么显示的不完全对呢~

#include <AT89X51.H>
unsigned char code dispcode[]={0x3f,0x06,0x5b,
0x66,0x6d,0x7d,0x07,
0x7f,0x6f,0x77,0x7c,
0x39,0x5e,0x79,0x71,0x00};
unsigned char dispbitcode[]={0xfe,0xfd,0xfb,0xf7,
0xef,0xdf,0xbf,0x7f};
unsigned char dispbuf[]={0,0,15,0,0,15,0,0};
unsigned char dispbitcnt;
unsigned char second=0;
unsigned char minite=0;
unsigned char hour=0;
unsigned int tcnt;
unsigned char mstcnt;
unsigned char i,j;
void main(void)
{
TMOD=0x02;
TH0=0x06;
TL0=0x06;
TR0=1;
ET0=1;
EA=1;
while(1)
{
if(P0_0==0)
{
for(i=5;i>0;i--)
for(j=248;j>0;j--);
if(P0_0==0)
{
second++;
if(second==60)
{
second=0;
}
dispbuf[0]=second%10;
dispbuf[1]=second/10;
while(P0_0==0);
}
}
if(P0_1==0)
{       
for(i=5;i>0;i--)
for(j=248;j>0;j--);
if(P0_1==0)
{
minite++;
if(minite==60)
{
minite=0;
}
dispbuf[3]=minite%10;
dispbuf[4]=minite/10;
while(P0_1==0);
}
}
if(P0_2==0)
{
for(i=5;i>0;i--)
for(j=248;j>0;j--);
if(P0_2==0)
{
hour++;
if(hour==24)
{
hour=0;
}
dispbuf[6]=hour%10;
dispbuf[7]=hour/10;
while(P0_2==0);
}
}
}
}
void t0(void) interrupt 1 using 0
{
mstcnt++;
if(mstcnt==8)
{
mstcnt=0;
P1=dispcode[dispbuf[dispbitcnt]];
P3=dispbitcode[dispbitcnt];
dispbitcnt++;
if(dispbitcnt==8)
{
dispbitcnt=0;
}
}
tcnt++;
if(tcnt==4000)
{
tcnt=0;
second++;
if(second==60)
{
second=0;
minite++;
if(minite==60)
{
minite=0;
hour++;
if(hour==24)
{
hour=0;
}
}
}
dispbuf[0]=second%10;
dispbuf[1]=second/10;
dispbuf[3]=minite%10;
dispbuf[4]=minite/10;
dispbuf[6]=hour%10;
dispbuf[7]=hour/10;       
}
}

为什么时间是从0走到70，而且每次7-8-9-A-10······17-18-19-1A-20····一直是这样怎么回事呢?大神们求指教