Excel VBA夏令时判断函数始终返回False的技术求助

Hey there! Let's fix that misbehaving function step by step—there are a couple of key issues causing it to always return False:

1. Critical Variable Name Typo (The Immediate Culprit) #

Look closely at your calling code:

isvaldst = isdaydat(Me.dtefrm.Value)
If isvaldat = True Then ' <-- Here's the problem!
    Me.dayconf.Value = "Daylight"
Else
    Me.dayconf.Value = "Regular"
End If

You assigned the function's result to isvaldst, but then checked against isvaldat (a completely different, undeclared variable). Since isvaldat defaults to Empty, the condition isvaldat = True never evaluates to true—hence you always see "Regular".

Fix: Correct the variable name in your If statement:

isvaldst = isdaydat(Me.dtefrm.Value)
If isvaldst = True Then
    Me.dayconf.Value = "Daylight"
Else
    Me.dayconf.Value = "Regular"
End If

2. Ambiguous Date Parsing (Potential Hidden Issue) #

Your function uses CDate("3/9/2008"), which relies on your system's regional date settings. If your system uses dd/mm/yyyy format (common in Europe, Australia, etc.), this will be parsed as September 3, 2008 instead of March 9, 2008—completely shifting your date range.

Fix: Use DateSerial(year, month, day) to create dates unambiguously, regardless of regional settings:

Public Function isdaydat(ByVal datval As Date) As Boolean
    Select Case True
        Case datval > DateSerial(2008, 3, 9) And datval < DateSerial(2008, 11, 1)
            isdaydat = True
        Case Else
            isdaydat = False
    End Select
End Function

3. Bonus Optimizations #

To make your code cleaner and more robust:

• Add Option Explicit at the top of your module—this forces you to declare all variables, catching typos like the one above immediately.
• Simplify the function by returning the boolean expression directly (no need for Select Case):

  Public Function isdaydat(ByVal datval As Date) As Boolean
      isdaydat = (datval > DateSerial(2008, 3, 9) And datval < DateSerial(2008, 11, 1))
  End Function
  

• Pass the raw Date value instead of the formatted text from your textbox to avoid unnecessary type conversion:

  fixdat = CDate(Me.dtefrm.Value)
  Me.dtefrm.Value = Application.WorksheetFunction.Text(fixdat, "mm/dd/yyyy") ' Fixed "yyy" to "yyyy" for full year
  isvaldst = isdaydat(fixdat) ' Use the already-converted Date variable
  

内容的提问来源于stack exchange，提问作者brokenspiv