Update cell data when ANOTHER cell is filled in

Okay, see attached example -

Part 1) Need to Update a cells value - When ANOTHER cell has data input into it.

Ex:
-B4 currently contains "05/14/1999"
-C4 is currently blank
-When data is entered into C4 ("03/20/2000") >>>>> B4 will now be UPDATED to refelct "03/20/2000"


Part 2) Need to then ARCHIVE the OLD cell value of B4 ("05/14/1999") to a running string of cells within the SAME ROW (so that past dates are stored/recorded)

Ex:
-B4 is updated to "03/03/2000"
-B4's OLD data of "05/14/1999" is sent to cell AB4
-Then the NEXT time cell C4 has data input, and B4 is once again updated, "03/03/2000" is also archived/moved to the next open cell to the previous archive (cell AB4)
-So that next time it's updated, "03/03/2000" will be moved to cell AB5
-And so on, and so on as the date is updated every few months.

Is this possible? Im guessing it is relativly easy using tables, and I suppose I can restructure it to make that work, but ideally I would like to keep it all in one row, on one sheet....(I am making this for our secratary to help her with scheduling and am trying to keep it as clise to her current configuration/system as possible so that she won;t have much to learn in terms of differneces.

Anyway, let me know, and as always I appreciate any help or guidence on this.

Thank you,

Tyler