This is based on the date of Apple's announcement of a new headset.

An announcement is a press release, Apple Event, or other Apple communication that substantially describes the headset, including its features, price, and timeline for availability.

A headset refers to any new mainline headset, no matter its name or target audience (Apple Vision Pro, Apple Vision Air, Apple Vision, Apple Space Goggles, etc.) counts. New colors, configurations, or extremely minor refreshes of the existing Apple Vision Pro do not count.

If there are any unclear possible situations, please leave a comment and I will be happy to clarify!