Copy, Paste formula output from one cell to another cell with values

Hi Experts.

I have been breaking my head for last 2 days.

All I have is a One excel File for multiple people, to update various milestone "Target dates" and "Actual achieved dates". All I want to do is track any changes done to any "Target Date" or "Actual Achieved Date" (probably in a separate hidden sheet for audit trail purpose).
Underlying Excel/Data structure:
Project Ref Milestone # Target Dt Actual Dt
'001 Milestone 1 1-Jan-15 5-Jan-15
'001 Milestone 2 15-Mar-15 15-Mar-15
'001 Milestone 3 30-Apr-15 27-Apr-15
'002 Milestone 1 30-Dec-14 Blank
'002 Milestone 2 31-Mar-15 Blank
.....


If the dates are getting updated, I(As administrator) want to track
1. When the date was changed
2. What was the old value
3. What is the new value.


This may be possible using VBA codes. I am trying to see, if this can be achieved using one Formula or multiple Formulae. I went through multiple forums and people recommending "Tracking Changes" using Review -> Tracking -> Highlight Changes. I do not think that will fulfill my needs. I felt, I may be able to achieve this:
1. Build a hidden sheet
2. Use a Vlookup to update current "Target date" from the original sheet
3. copy and paste formula (Vlookup) output in to another column as values ( so that this does not get updated whenever a user changes the date)

Any suggestion/guidance will be helpful. Thanks!

Originally Posted by sourabhg98

Okay we can achieve this...
But first it would be good if you could attach your workbook over here.
You can do so by clicking on go advanced button at the foot of the typing area....and then selecting the paper clip icon from the icons above...

Sample File attached