I can't find anywhere that I used n for minutes. Where in my code do you believe you found it? Have I overlooked a typo somewhere and can't find it?
Hi Ossie,
you must use n for minutes, m is always for month in VBA.Format:

The problem with the US / UK dates arises only if we write a text into the cell, and that is what the OP does.
I have a German PC and I have a dot as date separator, today in Germany is the 14.05.2020 which is the 05/14/2020 in US. But that is not the point.
If we have real dates in the sheet, Excel converts the dates to the local format, regardless where you are on the world. And also CDate uses the local format to convert a text into a real date.
The next step to support the local date format at the users end is to use VBA.FormatDateTime instead of VBA.Format to fill the Textboxes. After that it works where ever you are.
Copy the code below into your sample Userform, run it and click the button.
Here is a screenshot from my German PC:

Here is one from an US virtual machine:

As you see it doesn't matter which local format the end user has.
Andreas.
Private Sub CommandButton1_Click()
Dim tmeTimeDifference As Date
'(Subtract the Sum of Start Date and Time from the Sum of Finish Date and Time)
tmeTimeDifference = Abs((CDate(Me.txtFinishDate) + CDate(Me.txtFinishTime)) _
- (CDate(Me.txtStartDate) + CDate(Me.txtStartTime)))
Me.txtTimeDifference = IIf(tmeTimeDifference >= 1, CLng(Int(tmeTimeDifference)) & " days ", "") & Format(tmeTimeDifference, "hh:nn:ss")
'Example: Write the Date variable into the cell! Never a string!
Range("B3") = tmeTimeDifference
Range("B3").NumberFormat = "d ""days"" hh:mm:ss"
End Sub
Private Sub UserForm_Initialize()
Dim F As Date, T As Date
'Fill some random dates
Range("A1") = Now + 5 * Rnd
Range("A2") = Now + 5 * Rnd
Range("A3").Formula = "=ABS(A2-A1)"
Range("A3").NumberFormat = "d ""days"" hh:mm:ss"
'Get the lower date into F, higher into T
If Range("A1") < Range("A2") Then
F = Range("A1")
T = Range("A2")
Else
F = Range("A2")
T = Range("A1")
End If
'Fill the Textboxes
Me.txtStartDate = FormatDateTime(F, vbShortDate)
Me.txtStartTime = FormatDateTime(F, vbLongTime)
Me.txtFinishDate = FormatDateTime(T, vbShortDate)
Me.txtFinishTime = FormatDateTime(T, vbLongTime)
End Sub