1.方法一
由于每月天数的范围为[28,31]，所以可以依次从31到28，利用isdate函数判断是否为合法日期，首次true则该值就为当月天数；当然也可以依次从28到31,同样利用isdate判断是否为合法日期，首次false则该值减1就为当月天数，实现函数分别为：
Function Getnum(iyear,imonth)
Dim i,idate
For i=31 to 28 step -1
idate = iyear & "-" & imonth & "-" & i
if isdate(idate) then
Getnum = i
Exit function
End if
Next
End function和
Function Getnum(iyear,imonth)
Dim i,idate
For i=28 to 32
idate = iyear & "-" & imonth & "-" & i
if not(isdate(idate)) then
Getnum = i - 1
Exit function
End if
Next
End function想想哪个更好点^_^
2.方法二
本方法是通过对月份天数进行观察而总结出规律：1,3,5,7,8,10,12为31天；4,6,9,11为20天；2月有时为28天，有时为29天。实现函数为：
Function Getnum(iYear,iMonth)
Select Case iMonth
Case 1, 3, 5, 7, 8, 10, 12
Getnum = 31
Case 4, 6, 9, 11
Getnum = 30
Case 2
If IsDate(iyear & "-" & imonth & "-29") Then
Getnum = 29
Else
Getnum = 28
End If
End Select
End Function3.附date.asp，使用方法一显示指定年份各月份天数，便于观察
<%
Rem 取得月份天数
Function Getnum(iyear,imonth)
Dim i,idate
For i=31 to 28 step -1
idate = iyear & "-" & imonth & "-" & i
if isdate(idate) then
Getnum = i
Exit function
End if
Next
End function
Rem 标记不同天数颜色，方便浏览
Function Numcolor(n)
Select case n
case 28
Numcolor="<b><font color=red>" & n & "</font></b>"
case 29
Numcolor="<b><font color=blue>" & n & "</font></b>"
case 30
Numcolor="<b><font color=black>" & n & "</font></b>"
case 31
Numcolor="<b><font color=green>" & n & "</font></b>"
End select
End function
Dim n,j,k
Rem 1.表头显示1月到12月
Response.write "<table width='750' border='0' cellspacing='0' cellpadding='0'>" & vbcrlf
Response.write " <tr>" & vbcrlf
Response.write " <td width='50' height='20' align='center' valign='middle'> </td>" & vbcrlf
For n=1 to 12
Response.write " <td width='50' height='20' align='center' valign='middle'>" & n & "月</td>" & vbcrlf
Next
Response.write " </tr>" & vbcrlf
Rem 2.显示各年份中每月天数，本例年份从500至600
For j=500 to 600
Response.write " <tr>" & vbcrlf
Response.write " <td width='50' height='20' align='center' valign='middle'>" & j & "</td>" & vbcrlf
For k=1 to 12
Response.write " <td width='50' height='20' align='center' valign='middle'>" & Numcolor(Getnum(j,k)) & "</td>" & vbcrlf
Next
Response.write " </tr>" & vbcrlf
Next
Rem 3.表格结尾标识
Response.write "</table>"
%>
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