Ending a macro with the [X] button

[Nyanko]

I'm using the following code to control dates entered as a search procedure, using a calendar as a date entry. However I'd like the [X] button to exit the procedure, and can't figure out how ....

Public Sub GetValidDates(ByRef FromDate As Date, _
        ByRef ToDate As Date, _
        Optional MinDate As Date, _
        Optional MaxDate As Date)
    Dim bError As Boolean
    Dim sErrorMessage As String
    Dim vAbsenceStart As Variant, vAbsenceEnd As Variant

    '----------------------------------------------------------------------------
    ' Code below taken with advice from
    ' http://www.mrexcel.com/forum/showthread.php?t=274520
    '----------------------------------------------------------------------------

    vAbsenceStart = ""
    vAbsenceEnd = ""
    Do
        bError = False
        sErrorMessage = ""
        frmCalendarStart.Show
        vAbsenceStart = CalendarVal
        
        If IsDate(vAbsenceStart) = False Then
            bError = True
            vAbsenceStart = ""
            sErrorMessage = "Start date not a date"
        Else
            If CheckDateInRange(Datex:=vAbsenceStart, _
                    MinDate:=MinDate, _
                    MaxDate:=MaxDate) = False Then
                bError = True
                sErrorMessage = "Month start Date is not in range"
                vAbsenceStart = ""
            End If

            If bError = False Then
            frmCalendarEnd.Show
            vAbsenceEnd = CalendarVal

                If IsDate(vAbsenceEnd) = False Then
                    bError = True
                    sErrorMessage = "End date is not a date"
                    vAbsenceEnd = ""
                End If
            End If

            If bError = False Then
                If CheckDateInRange(Datex:=vAbsenceEnd, _
                        MinDate:=MinDate, _
                        MaxDate:=MaxDate) = False Then
                    bError = True
                    sErrorMessage = "End Date is not in range"
                    vAbsenceEnd = ""
                End If
            End If

            If vAbsenceEnd < vAbsenceStart Then
                bError = True
                sErrorMessage = "Start Date not before End date"
                vAbsenceEnd = ""
            End If

        End If
        If bError Then MsgBox Prompt:=sErrorMessage, Buttons:=vbOKOnly + vbCritical, Title:="Invalid Date!"
    Loop While bError

    FromDate = CDate(vAbsenceStart)
    ToDate = CDate(vAbsenceEnd)
End Sub
Private Function CheckDateInRange(ByVal Datex As Variant, _
        Optional MinDate As Date, _
        Optional MaxDate As Date) As Boolean
    '----------------------------------------------------------------------------
    ' Return False if specified date is not in range
    '----------------------------------------------------------------------------
    Dim datCur As Date

    On Error GoTo labError
    datCur = CDate(Datex)
    If Not (IsMissing(MinDate)) Then
        If datCur < MinDate Then
            CheckDateInRange = False
            Exit Function
        End If
    End If

    If Not (IsMissing(MaxDate)) Then
        If datCur > MaxDate Then
            CheckDateInRange = False
            Exit Function
        End If
    End If

    CheckDateInRange = True
    Exit Function

labError:
    CheckDateInRange = False
End Function