I'm presuming your title change is acceptable since it now depicts your need accurately.
I am also presuming the sheet you've uploaded is a "template" that must be initialized each week, thus your issue with when it is opened. Adding in a macro will require your users to enable macros on their system or at least when they open the sheet.
So, if this IS a template, first...UNLOCK cell B1 so that it is editable, clear the cell so that it is empty....then lock your sheet.
On the Menu Bar just to the left of FILE : EDIT : VIEW : INSERT you will see an Excel icon. Right-click on it and select VIEW CODE. This will open the WorkBook Module for you.
Now, paste in this code:
This macro will see the empty cell, add the commencement date, convert it to a real value, and lock the cell so it is no longer changeable.
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