I need to capture and date-stamp the contents of selective cells as they stood at the end of users previous session. Once the info is captured and date-stamped, I need to feed this data into a chart showing linear progression. Can I take a date-stamped snapshot of selected cells, and store that data in other cells?

Here's the scenario:

purpose of spread sheet:
PROJECT MANAGEMENT
TASKS ASSIGNED TO EMPLOYEES - With [Column C] showing % of Completion for Each Task - I want to track & store all changes made to [Column C] by date.
A B C D
[DESCRIPTION OF TASK] [ASSIGN TO] [PCT % COMPLETION] [DUE DATE]
1 Design New Web Page Layout Cindi 30% 04-APR-13
2 Studio A - Install A/V Equipment Bill 15% 05-MAY-13
3 Test Hardware/Software Connectivity Truman 25% 30-APR-13
4 Studio B - Tear out for Remodel Subcontractor 0% 15-JUN-13
5 Finalize Legal Agreements Grantham 75% 01-MAY-13


DESIRED CONTENT OF REPORT: (SAMPLE)
[EMPLOYEE] [TASK] [DATE1] [DATE 2] [DATE 3] [DATE 4] [DATE 5]
CINDI Design New Web Page Layout 0% 20% 25% 45% 66%
BILL Studio A - Install A/V Equipment 0% 0% 0% 15% 25%
TRUMAN Test Hard/Software Connectivity 10% 40% 100% 100% 100%
SUBCONTRACTOR Studio B - Tear out for Remodel 0% 0% 75% 100% 100%
GRANTHAM Finalize Legal Agreements 35% 35% 50% 50% 75%