Protecting a cell

I have this parking lot worksheet that is being used to track available spaces in a number of parking lots. I have a code that locks the enter sheet until a date is enter. What I need to happen is after the date is enter I need that 1 merged cell to be lock so no one can change the date. even if they go to a previous date I am enclosing a copy of the worksheet.Space Counts.xlsm I am also enclosing the code that lock the sheet until the date is entered. ("M10") is were the date goes.

Sub Auto_Open()

    Range("L9").Value = Now

    Application.OnTime Now + TimeValue("00:00:01"), "RunClock"

        Dim strFolder As String

        strFolder = "C:\Spaces\Space Counts.xlsm"

        Sheets("Sheet1").Range("M10").Select

    Dim res

    Do

    res = Application.InputBox("Enter The Date", "Hourly Spaces Counts")

        If IsDate(res) Then

        ActiveCell.Formula = DateValue(res)

    Exit Sub

    End If

        MsgBox "Invalid date", vbCritical, "Try again"

    Loop While Not IsDate(res)

End Sub

1.The Cell to be protected is in column 13 cell M10

2.The Worksheet is not protect

Thank You.

thank you