It is the 65th regnal year of Elizabeth II of England. Which is not to say she's been on the throne a full 65 years yet -- the typical convention is that the first partial year (unless crowned exactly on New Year's Day) is regnal year 1, the next, full year is regnal year 2, and so on. This was formerly a very common way of keeping track of calendars, and still retained today in some legal contexts -- rather than a continuous count from some abstract starting date irrelevant to most daily life, pay attention to the Power That Be. Most East Asian calendars were counted in finite eras that were often pegged directly to the reigns of the emperor or king -- this is still done in Japan, where 2015 is also Heisei 27, Heisei being the current emperor's imperial (rather than personal) name. Regnal was first used in English in 1612, adopted from Medieval Latin rēgnālis, from Latin rēgnum, rule/kingdom + an adjectival ending -- so a direct cognate of reign itself.
I have translated dates in all documents from a regnal year to the modern dating system.