Anniversary Date Calculator

An anniversary calculation has two halves. The elapsed interval is found by subtracting the start date from the current date component by component, borrowing days from the preceding month and 12 months from the year when a component goes negative. The next occurrence is found by placing the original month and day in the current year, then advancing one year if that date has already passed. The ordinal of that next occurrence is the difference between its year and the start year. A separate day count, taken from the millisecond difference between the two dates, drives cumulative milestones at 100, 365, 1,000 and 10,000 days. The traditional material list maps each year to a gift: paper for the first, cotton second, leather third, wood fifth, tin or aluminium tenth, crystal fifteenth, china twentieth, silver twenty-fifth, pearl thirtieth, ruby fortieth, gold fiftieth and diamond sixtieth. Years with no entry, which is most of them past twenty, have no traditional assignment at all, and modern lists published by retailers assign different materials to the same years, so the two systems frequently disagree. The one structural edge case is a 29 February start date, which has no anniversary in common years; most contracts and policies treat 1 March, or in some jurisdictions 28 February, as the equivalent.

The custom is older than the year by year list. Silver and gold wedding celebrations are documented in central Europe well before the modern era: in the German speaking lands a husband would crown his wife with a silver garland after twenty-five years of marriage and a gold one after fifty, giving the terms silberne Hochzeit and goldene Hochzeit. Only those two milestones, later joined by diamond, carried material associations for centuries. Diamond became attached to sixty years in Britain largely through Queen Victoria, whose Golden Jubilee in 1887 and Diamond Jubilee in 1897 fixed the pairing in popular usage. The dense modern list, with a material for every one of the first twenty years, is a twentieth century commercial construction. The American National Retail Jewelers Association published an expanded schedule in 1937 that filled the gaps between the traditional milestones, and etiquette writers including Emily Post circulated versions of it to a general readership. Retailers and greeting card companies later added parallel modern lists substituting clocks, china and appliances for the older materials, along with flower and gemstone equivalents. Because no standards body owns any of these lists, published versions differ, and day count milestones such as the 100 day marker popular in Korean dating culture are more recent still.