Anniversary calculator

The count decomposes two calendar dates, so it knows nothing about time zones or hours in the day. Milestones are every yearly anniversary plus round numbers: the 100th and 500th day then every 1,000th day, the 50th week then every 100th week, the 10th and 50th month then every 100th month, capped at 60 per export. Gift names come from published US wedding-anniversary lists; other lists name some years differently. The .ics file is all-day events with a fixed timestamp so the same inputs always produce the same file.