这是一个计算2000年01月01日起到指定日的天数的函数
<br>
(算头也算尾)日期格式为：YYYY-MM-DD
<br>

<?php
//计算从2000年01月01日开始到某日的天数

function cal_start2end($end_day,$start_day)
{


$start_day=ereg_replace("-","",$start_day);
$end_day=ereg_replace("-","",$end_day);



if($end_day>=$start_day)
{//截止日期大于开始日期
if(substr($end_day,0,4)==substr($start_day,0,4))
{

if( is_int(substr($end_day,0,4)/4) )
$leap_day=29;//leap year
else
$leap_day=28;


////同一年

if(substr($end_day,4,2)==substr($start_day,4,2))
{
//the same month
$endday_from_startday=$end_day-$start_day+1;

}
else
{
//not same month


switch (substr($end_day,4,2))
{
case "01":
$endday_from_newyear=substr($end_day,6,2)+1;
break;
case "02":
$endday_from_newyear=substr($end_day,6,2)+31;
break;
case "03":
$endday_from_newyear=substr($end_day,6,2)+$leap_day+31;
break;
case "04":
$endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*2;
break;
case "05":
$endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*2+30;
break;
case "06":
$endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*3+30;
break;
case "07":
$endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*2+30*2;
break;
case "08":
$endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*3+30*2;
break;
case "09":
$endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*5+30*2;
break;
case "10":
$endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*5+30*3;
break;
case "11":
$endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*5+30*4;
break;
case "12":
$endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*5+30*5;
break;
}
$endday_from_startday=$endday_from_newyear;
}

return ($endday_from_startday);

}
else
{
//不同年！

$differ_year=substr($end_day,0,4)-substr($start_day,0,4);

$how_int_4_floor=floor($differ_year/4)+1; // 向下取整
$how_int_4_ceil=ceil($differ_year/4)+1; // 向上取整



if($how_int_4_floor==$how_int_4_ceil)
$how_leap_year=$how_int_4_floor-1;
else
$how_leap_year=$how_int_4_floor;

$how_noleap_year=$differ_year-$how_leap_year;

$differ_year2day=$how_noleap_year*365+$how_leap_year*366;



if( is_int(substr($end_day,0,4)/4) )
$leap_day=29;//leap year
else
$leap_day=28;



if(substr($end_day,4,2)==substr($start_day,4,2))
{
//the same month
$endday_from_startday=$differ_year2day+ substr($end_day,4,2);

}
else
{
//not same month


switch (substr($end_day,4,2))
{
case "01":
$endday_from_newyear=substr($end_day,6,2)+1;
break;
case "02":
$endday_from_newyear=substr($end_day,6,2)+1+31;
break;
case "03":
$endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31;
break;
case "04":
$endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*2;
break;
case "05":
$endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*2+30;
break;
case "06":
$endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*3+30;
break;
case "07":
$endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*2+30*2;
break;
case "08":
$endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*3+30*2;
break;
case "09":
$endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*5+30*2;
break;
case "10":
$endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*5+30*3;
break;
case "11":
$endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*5+30*4;
break;
case "12":
$endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*5+30*5;
break;
} //End of switch


$endday_from_startday=$endday_from_newyear+$differ_year2day;
}//End ofnot same month



return ($endday_from_startday);


} //End of year
}//End of$end_day>=$start_day



} //End of function


$start_day="2000-01-01";

$end_day="2011-01-01";

echo "您输入的是：".$end_day."<br>\n";
$endday_from_startday=cal_start2end($end_day,$start_day);

echo "该日距离$start_day 共 ".$endday_from_startday." 天";

?>
<br><br><br><br>时间太少了，那位大虾可以修改为计算任意两日期之间的间隔最好！！！！！