This is a best practice kind of question, rather than a practical "how do I get x, y and z to happen".

I have a data source for our employees including their pay rates. Their pay rates change over time, not just once a year necessarily.

Each month I take the data from their time sheets (hours) which allocates time to our activities and pull in the rates from the data source. This is then available as a report on the cost of the various activities undertaken.

So, how do I ensure that the data for, say, January 2018 is correct if the data source rates change over time? Obviously I could just not refresh the query or the spreadsheet but what if I forget? What if someone else uses it and refreshes it?

Now, it happens that the data source at present is a spreadsheet table that I maintain more or less manually, so I could potentially keep multiple versions and change the source when I create the next month's report when the data changes to point to the newer version. And that is probably what I'll do, but I am interested to see if there is any kind of best practice approach (I did a google search but nothing seemed to come up).

Also, it is feasible that in the future the data source will become something I can't control, that is, it will come straight from our payroll system.