I've been asked to post a verse I composed on a spreadsheet for constructing a vertical decliner. Here it is, in a slightly revised version:
On September one, trust the sun. Come Halloween, subtract sixteen. On Christmas Day, you're OK. For your Valentine true, add a dozen and two. When taxes are due, the dial is true. At the mid of May, take four away. On June fourteen, don't add a bean. When August begins, add seven little mins. The rest is easy: for any date, All you do is interpolate. The line about taxes works for USA. A more universal alternate might run, The mid of month four, add no more.