Styjun

If the "dd" value of the date is "12" or less, on the StartDate variable, then the answer is incorrect.

Eg. When the StartDate is "03/12/19" and the EndDate value is "03/23/19", it yields -5573, which is obviously the incorrect answer.

If the "dd" value of the date is "13" or more, on the StartDate variable, then the answer is correct.

Eg When the StartDate is "03/13/19" and the EndDate value is "03/23/19", it yields 10, which is the correct answer.

I don't quite understand.
BTW the vba code is on a MacOS Excel Mac 2011.

Also, the weird thing is that this exact same workbook code works perfectly on my Win 7 PC Excel 2007 regardless of what value of the "dd" you use.!!!!

Sub DateDiffTest()

 Dim StartDate As Date ' Declare variables.
 Dim EndDate As Date ' Declare variables.
 Dim Msg
 StartDate = Format("03/12/19", "mm-dd-yy")
 EndDate = Format("03/23/19", "mm-dd-yy")
 Msg = "Days from today: " & DateDiff("d", EndDate, StartDate)
 MsgBox Msg
End Sub

Your problems have to do with undeclared variables, and with VBA being US-Centric in its use of dates and with your Regional settings being inconsistent with what you are doing.

I suspect your Regional short date settings are YMD.

And what you think is dd, VBA thinks is mm.

So when the value is <= 12 it converts it to YMD
StartDate --> 2003-Dec-19

When it is >12, it cannot be a month, so VBA interprets it as a day, and converts it as a US-Centric date. So EndDate --> 2019-Mar-23

You should declare all variables, and also, when coding, use unambiguous dates.

eg:

Option Explicit
Sub DateDiffTest()
 'OK to use US-centric MDY here
 Const StartDate As Date = #3/12/2019#
 Const EndDate As Date = #3/23/2019#
 Dim Msg As String

Msg = "Days from today: " & DateDiff("d", EndDate, StartDate)
MsgBox Msg
End Sub

Another method of unambiguously declaring the dates:

Option Explicit
Sub DateDiffTest()
 Dim StartDate As Date
 Dim EndDate As Date
 Dim Msg As String

StartDate = "2019-Mar-12"
EndDate = "2019-Mar-23"

Msg = "Days from today: " & DateDiff("d", EndDate, StartDate)
MsgBox Msg
End Sub

By the way, I don't understand why MS makes NOT requiring variable declaration the default. Select Tools/Options/Editor and check Require Variable Declaration. This will place Option Explicit at the start of any new module. To correct this module, enter it manually at the beginning.