Date of Data Entry / Transaction

Hi All,

I ahve a tricky one and it should be possible but i have not come accross the implict calculation yet.

The situation is this:
We have multiple employees entrying data into a Excel Tables which is on a common drive, these tables are linked to various reports that help us communicate more effectively. 

The Question:
How can i best determine what date and time of each transaction was entered / captured. This should be done without the data capturer entering any additional information, people lie... 

So Far:
I have just used simple dates to termined how many transactions are done daily.
Really appreciate any help with this.