Excel vba error with dates on sheet and textbox

There is some confusion with strings that look like dates and actual dates and the EN-US-centric VBA default MDY locale is doing nothing to alleviate that.

First off, the date in A1 is a number between 0 and 42,225 (Aug 9, 2015 is 42,225); nothing more or less. You can dress it up as d/mm/yyyy regardless of what locale your computer system is running under. Mine runs under EN-US and I have no trouble displaying a date as 9/08/2015 if I use a number format mask of d/mm/yyyy. To get the displayed date from A1 back into the textbox AS A STRING use the Range.Text property. This is the displayed text that is in a cell. It is read-only so you cannot stuff a "9/08/2015" string back into it.

Private Sub UserForm_Initialize()
    DateCurrent_Tbx.Value = ThisWorkbook.Sheets(1).Range("A1").TEXT
End Sub

As far as user input of a 'string-that-looks-like-a-date' is concerned, you may have to trust your users a bit to do the right thing. If they can be relied upon to input in a DMY format, you can split the pieces and put them together properly using the DateSerial function.

Private Sub LockInput_Cmd_Click()
    dim dt as date, vDT as variant
    vDT = split(InputDate_Tbx.Value, chr(47))  'split the string-date on the forward slash
    dt = DateSerial(vDT(2), vDT(1), vDT(0))   'create a real date
    SetDate_Cmd.Caption = Format(dt, "d/mmm/yy")   'use any format mask you want to create another string-date
End Sub

Private Sub SetDate_Cmd_Click()
    dim dt as date, vDT as variant
    vDT = split(SetDate_Cmd.Caption, chr(47))  'split the string-date on the forward slash
    dt = DateSerial(vDT(2), vDT(1), vDT(0))   'create a real date
    ThisWorkbook.Sheets(1).Range("A1").Value = dt  'put the actual date back into A1
End Sub

As far as the CDate function is concerned, I would avoid its use. If you stuff "19/08/2015" into CDate (try it in the VBE's Immediate window as ?CDate("19/08/2015")) it will correctly interpret it as 19-Aug-2015 but that is only because IT HAS NO OTHER CHOICE! If you try the same with ?CDate("9/08/2015") you end up with 08-Sep-2015 because whenever there is a choice, CDate opts for the EN-US locale of MDY. Best not to rely on this one. I believe the DateValue function is at least as unreliable.