Temporal calculations—that is, calculations based on dates and times—are often hard to put together. One reason they're so tricky is that more than one convention exists for expressing starting and ending dates and times. For example, we generally accept that January is the first month of the year and that the first month of the year has 31 days. However, in some financial calculations, October might be the first month of the fiscal year, or all months might have a fixed length of 30 d...