Adding years is the simplest of these operations and has 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 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 one weekday each year and two whenever a 29 February falls in between, which is why an anniversary returns to the same day of the week after five, six or eleven years rather than on any tidy cycle. The weekday of the result is shown, since that is usually what someone planning around a future date actually wants.
Whole-year terms are common in the UAE — residence permits issued for two, five or ten years, tenancy renewals, multi-year service agreements — and the expiry weekday decides when the paperwork can realistically be dealt with. For long spans the total number of days is also given, including the leap days crossed; that figure pulls away from a plain multiple of 365 by about one day every four years, so an estimate built on the round number is a week adrift within a few decades.
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