I suppose it’s redundant if your editor stores, or at least references, an actual date for each line, but if not, this is the way to go:
How about an editor that color codes the age of particular lines of source code? You have a piece of source that has been around for a while, either it’s under development or it needs some changes. Wouldn’t be nice if the older lines, lines of code that have proven themselves to be useful and correct were given a dark gray background, newer lines could be given a white background, and lines that have been changed umpteen zillion times a red background?
“Piece of source,” at least in some shops I could name, is usually shortened to “POS,” as in “Who the hell added that extra loop into this POS?”