My Apologise,
Doe this help?

Private Sub cmdcalculate_Click()

TextBox1 = TextBox1.Text
TextBox2 = TextBox2.Text

TextBox3 = Format(CDate(TextBox2) - CDate(TextBox1), "h:mm")

End Sub

Private Sub cmdclearinfo_Click()

 TextBox1.Value = ""
 TextBox2.Value = ""
 TextBox3.Value = ""
 
 
End Sub

Private Sub cmdexist_Click()
  Unload UserForm1
End Sub


Private Sub TextBox1_BeforeUpdate(ByVal Cancel As MSForms.ReturnBoolean)
   
   If Not Me.TextBox1 Like "??:??" Then
     MsgBox "Please use format 'hh:mm'"
     Cancel = True
     Exit Sub
   Else
   
  End If
     
   myVar = Application.WorksheetFunction.Text(Me.TextBox1, "hh:mm am/pm")
   
   Me.TextBox1 = myVar
   
End Sub

Private Sub TextBox2_BeforeUpdate(ByVal Cancel As MSForms.ReturnBoolean)
If Not Me.TextBox2 Like "??:??" Then
     MsgBox "Please use format 'hh:mm'"
     Cancel = True
     Exit Sub
   Else
   
  End If
     
   myVar = Application.WorksheetFunction.Text(Me.TextBox2, "hh:mm am/pm")
   
   Me.TextBox2 = myVar
End Sub