An honest scientific discussion would look at the degree of variation over time, say hundreds or thousands of years, and then put recent variations in this context. (climate-skeptic.com)
A further problem in the American context is that there is little recent variation in the length of school year. (educationnext.org)
A more recent variation on this technique, with even more benefits to the seller, is the structured installment sale, based on Section 453 of the Internal Revenue Code. (realtormag.realtor.org)