Validate user-entered dates in Classic ASP

A form can collect year, month and day separately, validate them on the server and then build a Date value. Avoid constructing an ambiguous string such as yyyy/mm/dd and assuming every server locale will parse it the same way.

Complete Classic ASP example

<%@ Language=VBScript %>
<% Option Explicit %>
<%
Dim submitted, monthNo, dayNo, yearNo
Dim myDate, validDate

submitted = (Request.Form("todo") = "submit")

If submitted Then
  monthNo = Request.Form("month")
  dayNo = Request.Form("day")
  yearNo = Request.Form("year")
  validDate = False

  If IsNumeric(monthNo) And IsNumeric(dayNo) And IsNumeric(yearNo) Then
    monthNo = CInt(monthNo)
    dayNo = CInt(dayNo)
    yearNo = CInt(yearNo)

    If monthNo >= 1 And monthNo <= 12 And _
       dayNo >= 1 And dayNo <= 31 And _
       yearNo >= 100 And yearNo <= 9999 Then
      myDate = DateSerial(yearNo, monthNo, dayNo)
      validDate = (Year(myDate) = yearNo And Month(myDate) = monthNo And Day(myDate) = dayNo)
    End If
  End If

  If validDate Then
    Response.Write "<p>Valid date: " & Server.HTMLEncode(CStr(myDate)) & "</p>"
  Else
    Response.Write "<p>Please enter a valid date.</p>"
  End If
End If
%>
<form method="post" action="">
  <input type="hidden" name="todo" value="submit">
  <label>Month <select name="month" required>
  <% Dim m : For m = 1 To 12 %>
    <option value="<%=m%>"><%=MonthName(m)%></option>
  <% Next %>
  </select></label>
  <label>Day <select name="day" required>
  <% Dim d : For d = 1 To 31 %>
    <option value="<%=d%>"><%=d%></option>
  <% Next %>
  </select></label>
  <label>Year <input type="number" name="year" min="100" max="9999" value="2026" required></label>
  <button type="submit">Validate date</button>
</form>

The Year/Month/Day comparison after DateSerial() is important because DateSerial() normalizes overflow. For example, February 31 rolls into March instead of raising a validation error.

Subhendu Mohapatra

Author & Instructor at plus2net

I write and maintain practical tutorials on Python, PHP, SQL, JavaScript, HTML, jQuery, and web development at plus2net. The tutorials focus on clear explanations, working examples, and code that readers can test and adapt while learning.

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