Adding years is the simplest of these operations and has exactly one difficult case: 29 February. Add a year to a leap day and there is no 29 February to land on, so the result falls back to 28 February. Add four years and the date reappears intact. Every other date keeps its day and month and simply changes the year.
The weekday, by contrast, never stays put. A date advances by one weekday each year and by two whenever a 29 February intervenes, which is why an anniversary returns to the same day of the week after five, six or eleven years rather than on any regular seven-year cycle. The calculator shows the weekday for the result, which is the part people actually want when they are looking at a golden wedding, a lease expiry or a twenty-fifth anniversary several years out.
For long spans it also reports the total number of days, including the leap days crossed. That figure diverges from a multiple of 365 quite quickly — around one day every four years — so any estimate built on the round number will be a week adrift over a few decades. Negative values run the same arithmetic backwards, which is the usual way of finding a birth year from an age at a known date.
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