That particular strip, too, was one drawn at least a couple of years before, and saved. And in that case, the strip was "blank"- no dialogue. I had drawn maybe half a dozen like that, meant for those busy days when I needed a last-minute strip. The "Watch for Falling Rocks" strip was done at about the same time, as well as the one where Doc was doing something just off panel while Sandy grouses about something- the last panel is Doc about to swing a sledgehammer and saying "We're being very careful".
So despite the fact that strip is dated 2012 (and posted in 2013) I'm pretty sure it's a year or two older than that.