IsDate function returns unexpected results

I got tripped up by this little "feature" recently and wanted to raise awareness of some of the issues surrounding the IsDate function in VB and VBA.

The Simple Case

As you'd expect, IsDate returns True when passed a Date data type and False for all other data types except Strings. For Strings, IsDate returns True or False based on the contents of the string:

IsDate(CDate("1/1/1980"))  --> TrueIsDate(#12/31/2000#)       --> TrueIsDate(12/24)              --> False  '12/24 evaluates to a Double: 0.5'IsDate("Foo")              --> FalseIsDate("12/24")            --> True

IsDateTime?

IsDate should be more precisely named IsDateTime because it returns True for strings formatted as times:

IsDate("10:55 AM")   --> TrueIsDate("23:30")      --> True  'CDate("23:30")   --> 11:30:00 PM'IsDate("1:30:59")    --> True  'CDate("1:30:59") --> 1:30:59 AM'IsDate("13:55 AM")   --> True  'CDate("13:55 AM")--> 1:55:00 PM'IsDate("13:55 PM")   --> True  'CDate("13:55 PM")--> 1:55:00 PM'

Note from the last two examples above that IsDate is not a perfect validator of times.

The Gotcha!

Not only does IsDate accept times, it accepts times in many formats. One of which uses a period (.) as a separator. This leads to some confusion, because the period can be used as a time separator but not a date separator:

IsDate("13.50")     --> True  'CDate("13.50")    --> 1:50:00 PM'IsDate("12.25")     --> True  'CDate("12.25")    --> 12:25:00 PM'IsDate("12.25.10")  --> True  'CDate("12.25.10") --> 12:25:10 PM'IsDate("12.25.2010")--> False '2010 > 59 (number of seconds in a minute - 1)'IsDate("24.12")     --> False '24 > 23 (number of hours in a day - 1)'IsDate("0.12")      --> True  'CDate("0.12")     --> 12:12:00 AM

This can be a problem if you are parsing a string and operating on it based on its apparent type. For example:

Function Bar(Var As Variant)    If IsDate(Var) Then        Bar = "This is a date"    ElseIf IsNumeric(Var) Then        Bar = "This is numeric"    Else        Bar = "This is something else"    End IfEnd Function?Bar("12.75")   --> This is numeric?Bar("12.50")   --> This is a date

The Workarounds

If you are testing a variant for its underlying data type, you should use TypeName(Var) = "Date" rather than IsDate(Var):

TypeName(#12/25/2010#)  --> DateTypeName("12/25/2010")  --> StringFunction Bar(Var As Variant)    Select Case TypeName(Var)    Case "Date"        Bar = "This is a date type"    Case "Long", "Double", "Single", "Integer", "Currency", "Decimal", "Byte"        Bar = "This is a numeric type"    Case "String"        Bar = "This is a string type"    Case "Boolean"        Bar = "This is a boolean type"    Case Else        Bar = "This is some other type"    End SelectEnd Function?Bar("12.25")   --> This is a string type?Bar(#12/25#)   --> This is a date type?Bar(12.25)     --> This is a numeric type

If, however, you are dealing with strings that may be dates or numbers (eg, parsing a text file), you should check if it's a number before checking to see if it's a date:

Function Bar(Var As Variant)    If IsNumeric(Var) Then        Bar = "This is numeric"    ElseIf IsDate(Var) Then        Bar = "This is a date"    Else        Bar = "This is something else"    End IfEnd Function?Bar("12.75")   --> This is numeric?Bar("12.50")   --> This is numeric?Bar("12:50")   --> This is a date

Even if all you care about is whether it is a date, you should probably make sure it's not a number:

Function Bar(Var As Variant)    If IsDate(Var) And Not IsNumeric(Var) Then        Bar = "This is a date"    Else        Bar = "This is something else"    End IfEnd Function?Bar("12:50")   --> This is a date?Bar("12.50")   --> This is something else

Peculiarities of CDate

As @Deanna pointed out in the comments below, the behavior of CDate() is unreliable as well. Its results vary based on whether it is passed a string or a number:

?CDate(0.5)     -->  12:00:00 PM?CDate("0.5")   -->  12:05:00 AM

Trailing and leading zeroes are significant if a number is passed as a string:

?CDate(".5")    -->  12:00:00 PM ?CDate("0.5")   -->  12:05:00 AM ?CDate("0.50")  -->  12:50:00 AM ?CDate("0.500") -->  12:00:00 PM 

The behavior also changes as the decimal part of a string approaches the 60-minute mark:

?CDate("0.59")  -->  12:59:00 AM ?CDate("0.60")  -->   2:24:00 PM 

The bottom line is that if you need to convert strings to date/time you need to be aware of what format you expect them to be in and then re-format them appropriately before relying on CDate() to convert them.

Late to the game here (mwolfe02 answered this a year ago!) but the issue is still real, there are alternative approaches worth investigating, and StackOverflow is the place to find them: so here's my own answer...

I got tripped up by VBA.IsDate() on this very issue a few years ago, and coded up an extended function to cover cases that VBA.IsDate() handles badly. The worst one is that floats and integers return FALSE from IsDate, even though date serials are frequently passed as Doubles (for DateTime) and Long Integers (for dates).

A point to note: your implementation might not require the ability to check array variants. If not, feel free to strip out the code in the indented block that follows Else ' Comment this out if you don't need to check array variants. However, you should be aware that some third-party systems (including realtime market data clients) return their data in arrays, even single data points.

More information is in the code comments.

Here's the Code:

Public Function IsDateEx(TestDate As Variant, Optional LimitPastDays As Long = 7305, Optional LimitFutureDays As Long = 7305, Optional FirstColumnOnly As Boolean = False) As Boolean'Attribute IsDateEx.VB_Description = "Returns TRUE if TestDate is a date, and is within ± 20 years of the system date.'Attribute IsDateEx.VB_ProcData.VB_Invoke_Func = "w\n9"Application.Volatile FalseOn Error Resume Next' Returns TRUE if TestDate is a date, and is within ± 20 years of the system date.' This extends VBA.IsDate(), which returns FALSE for floating-point numbers and integers' even though the VBA Serial Date is a Double. IsDateEx() returns TRUE for variants that' can be parsed into string dates, and numeric values with equivalent date serials.  All' values must still be ±20 years from SysDate. Note: locale and language settings affect' the validity of day- and month names; and partial date strings (eg: '01 January') will' be parsed with the missing components filled-in with system defaults.' Optional parameters LimitPastDays/LimitFutureDays vary the default ± 20 years boundary' Note that an array variant is an acceptable input parameter: IsDateEx will return TRUE' if all the values in the array are valid dates: set  FirstColumnOnly:=TRUE if you only' need to check the leftmost column of a 2-dimensional array.' *     THIS CODE IS IN THE PUBLIC DOMAIN' *' *     Author: Nigel Heffernan, May 2005' *     http://excellerando.blogspot.com/' *' *' *     *********************************Dim i As LongDim j As LongDim k As LongDim jStart As LongDim jEnd   As LongDim dateFirst As DateDim dateLast As DateDim varDate As VariantdateFirst = VBA.Date - LimitPastDaysdateLast = VBA.Date + LimitFutureDaysIsDateEx = FalseIf TypeOf TestDate Is Excel.Range Then    TestDate = TestDate.Value2End IfIf VarType(TestDate) < vbArray Then    If IsDate(TestDate) Or IsNumeric(TestDate) Then        If (dateLast > TestDate) And (TestDate > dateFirst) Then            IsDateEx = True        End If    End IfElse   ' Comment this out if you don't need to check array variants    k = ArrayDimensions(TestDate)    Select Case k    Case 1        IsDateEx = True        For i = LBound(TestDate) To UBound(TestDate)            If IsDate(TestDate(i)) Or IsNumeric(TestDate(i)) Then                If Not ((dateLast > CVDate(TestDate(i))) And (CVDate(TestDate(i)) > dateFirst)) Then                    IsDateEx = False                    Exit For                End If            Else                IsDateEx = False                Exit For            End If        Next i    Case 2        IsDateEx = True        jStart = LBound(TestDate, 2)        If FirstColumnOnly Then            jEnd = LBound(TestDate, 2)        Else            jEnd = UBound(TestDate, 2)        End If        For i = LBound(TestDate, 1) To UBound(TestDate, 1)            For j = jStart To jEnd                If IsDate(TestDate(i, j)) Or IsNumeric(TestDate(i, j)) Then                    If Not ((dateLast > CVDate(TestDate(i, j))) And (CVDate(TestDate(i, j)) > dateFirst)) Then                        IsDateEx = False                        Exit For                    End If                Else                    IsDateEx = False                    Exit For                End If            Next j        Next i    Case Is > 2        ' Warning: For... Each enumerations are SLOW        For Each varDate In TestDate            If IsDate(varDate) Or IsNumeric(varDate) Then                If Not ((dateLast > CVDate(varDate)) And (CVDate(varDate) > dateFirst)) Then                    IsDateEx = False                    Exit For                End If            Else                IsDateEx = False                Exit For            End If        Next varDate    End SelectEnd IfEnd Function

A Tip for people still using Excel 2003:

If you (or your users) are going to call IsDateEx() from a worksheet, put these two lines in, immediately below the function header, using a text editor in an exported .bas file and reimporting the file, because VB Attributes are useful, but they are not accessible to the code editor in Excel's VBA IDE:

Attribute IsDateEx.VB_Description = "Returns TRUE if TestDate is a date, and is within ± 20 years of the system date.\r\nChange the defaulte default ± 20 years boundaries by setting values for LimitPastDays and LimitFutureDays\r\nIf you are checking an array of dates, ALL the values will be tested: set FirstColumnOnly TRUE to check the leftmost column only."

That's all one line: watch out for line-breaks inserted by the browser! ...And this line, which puts isDateEX into the function Wizard in the 'Information' category, alongside ISNUMBER(), ISERR(), ISTEXT() and so on:

Attribute IsDateEx.VB_ProcData.VB_Invoke_Func = "w\n9"

Use "w\n2" if you prefer to see it under the Date & Time functions: beats hell outta losing it in the morass of 'Used Defined' functions from your own code, and all those third-party add-ins developed by people who don't do quite enough to help occasional users.

I have no idea whether this still works in Office 2010.

Also, you might need the source for ArrayDimensions:

This API declaration is required in the module header:

Private Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" _                   (Destination As Any, _                    Source As Any, _                    ByVal Length As Long)

…And here's the function itself:

Private Function ArrayDimensions(arr As Variant) As Integer  '-----------------------------------------------------------------  ' will return:  ' -1 if not an array  ' 0  if an un-dimmed array  ' 1  or more indicating the number of dimensions of a dimmed array  '-----------------------------------------------------------------  ' Retrieved from Chris Rae's VBA Code Archive - http://chrisrae.com/vba  ' Code written by Chris Rae, 25/5/00  ' Originally published by R. B. Smissaert.  ' Additional credits to Bob Phillips, Rick Rothstein, and Thomas Eyde on VB2TheMax  Dim ptr As Long  Dim vType As Integer  Const VT_BYREF = &H4000&  'get the real VarType of the argument  'this is similar to VarType(), but returns also the VT_BYREF bit  CopyMemory vType, arr, 2  'exit if not an array  If (vType And vbArray) = 0 Then    ArrayDimensions = -1    Exit Function  End If  'get the address of the SAFEARRAY descriptor  'this is stored in the second half of the  'Variant parameter that has received the array  CopyMemory ptr, ByVal VarPtr(arr) + 8, 4  'see whether the routine was passed a Variant  'that contains an array, rather than directly an array  'in the former case ptr already points to the SA structure.  'Thanks to Monte Hansen for this fix  If (vType And VT_BYREF) Then    ' ptr is a pointer to a pointer    CopyMemory ptr, ByVal ptr, 4  End If  'get the address of the SAFEARRAY structure  'this is stored in the descriptor  'get the first word of the SAFEARRAY structure  'which holds the number of dimensions  '...but first check that saAddr is non-zero, otherwise  'this routine bombs when the array is uninitialized  If ptr Then    CopyMemory ArrayDimensions, ByVal ptr, 2  End IfEnd Function

Please keep the acknowledgements in your source code: as you progress in your career as a developer, you will come to appreciate your own contributions being acknowledged.

Also: I would advise you to keep that declaration private. If you must make it a public Sub in another module, insert the Option Private Module statement in the module header. You really don't want your users calling any function with CopyMemoryoperations and pointer arithmetic.