Copying Last Row Then Inserting It Screws Up Part Of Formula?

I have a spreadsheet serving as a simple document register. Column A is a unique document reference number, which is a simple concatenation of some text which prefixes all of the document numbers, plus the date in "yyymmdd" format (from column B) and a 3-digit sequence number (from column C).

Column B has no formula - my macro just pastes a timestamp in there.

Column C is a sequence number field which has a formula comparing the "yyymmdd" part of the timestamp with the cell directly above it. If they are different then the sequence number becomes "001" - if they are the same then the sequence number is incremented by 1, so all docs created on the same day have an incremental sequence number.

Here's the problem. When I want to add a new document I need to copy the last row down to the next row (in preparation for the next document to be created by my system) and there are two ways I've been doing this - one which works and one which doesn't.

This works > Say my last row (with the hidden formulae) is row 13. I select A13:C13 plus the three blank cells directly below (A14:C14) and do a Ctrl+D to copy them down. All is well.

But the following doesn't work - why? > I select the entire row 13, copy it, then reinsert it at row 13 so the formulae in row 13 get shifted down to row 14. What goes wrong here is that my formula in column C which WAS comparing the date in B13 to the one in B12 NOW compares B13 to B11

I've attached an example spreadsheet to try and demonstrate the problem. Sheet 1 is the original sheet. Sheet 2 is the "copying down" method which works, and Sheet 3 shows what happens when you copy the entire row 13 and "Insert copied cells".

excel-insertrow-formula-problem.xls