Automatically Update A Protected Cell Based On Contents Of An Unprotected Cell

I am trying to find some VBA that will allow protected cells to be updated when a user enters or selects data in other cells.

For example:

Cell 'A1' starts out blank.
Cell 'B1' has a drop down to select specific items.
Cell 'C1' is the date of the order and is a locked cell.
Cell 'D1' is the date of completion and is a locked cell.
The worksheet is protected to prevent direct changes to cells C1 and D1.

If a user enters any information in A1, then C1 displays the current date.
If a user selects 'Complete' from the drop-down list in B1, then D1 displays the current date. Both of these actions are independant of one another. This format is the same for every cell in the 4 columns indicated above. I thought I was able to do this in another spreadsheet I created a year or so ago but I have not been able to figure out what I did and I do not have the spreadsheet to look at.