刚刚测试了个键盘控制程序，其他按键都正常，但加法键一按就清0

#include<reg52.h>
#define uchar unsigned char
#define uint unsigned int
sbit led1=P1^0;
sbit dula=P2^6;
sbit key1=P3^4;
sbit key2=P3^5;
sbit key3=P3^6;
sbit key4=P3^7;
sbit wela=P2^7;//定义特殊功能寄存器
uchar num,numt0;
uchar code table[]={0x3f,0x06,0x5b,0x4f,0x66,0x6d,0x7d,0x07,0x7f,
0x6f,0x77,0x7c,0x39,0x5e,0x79,0x71};
void delayms(uint);
void display(uchar numdis)
{
uchar shi,ge;
shi=numdis/10;
ge=numdis%10;
dula=1;
P0=table[shi];
dula=0;
wela=1;
P0=0xfe;
wela=0;
delayms(5);//第一个数码管显示十位

dula=1;
P0=table[ge];
dula=0;
wela=1;
P0=0xfd;
wela=0;
delayms(5);//第二个数码管显示个位
}
void delayms(uint xms)
{
uint i,j;
for(i=xms;i>0;i--)
for(j=110;j>0;j--);
}

void keyscan()
{
if(key1==0)
{
delayms(10);
if(key1==0)
{
num++;
if(num==60);
num=0;
while(!key1);
}
}
if(key2==0)
{
delayms(10);
if(key2==0)
{
if(num==0)
num=60;
num--;
while(!key2);
}
}
if(key3==0)
{
delayms(10);
if(key3==0)
{
num=0;
while(!key3);
}

}
if(key4==0)
{
delayms(10);
if(key4==0)
{
while(!key4);
TR0=~TR0;
}
}
}
void main()
{

TMOD=0x01;
TH0=(65536-45872)/256;
TL0=(65536-45872)%256;
EA=1;
ET0=1;
while(1)
{
keyscan();
display(num);
}
}
void T0_time() interrupt 1
{
TH0=(65536-45872)/256;
TL0=(65536-45872)%256;
numt0++;
if(numt0==20)
{
numt0=0;
num++;
if(num==60)
num=0;
}
}