Birthday sats
Birthday sats are the rare sats mined on a chosen calendar day; the birthday page shows how many the index locates, the listings holding them and their sales.
What a sat's birthday is
Every sat was mined in a block, and every block on a UTC day: the day its header's timestamp falls on. That day is the sat's mint day, and its month and day are its birthday. Block timestamps are not strictly in order (a block may be dated before its parent), so a few blocks are filed under the day their own time says, and some early days of 2009 have no block at all.
The birthday page
The birthday page shows what the index tracks that was mined on a day:
/birthday/2009-01-03is one date: every category's tracked sats mined that day, how many of them the index locates, the listings here holding one, and their sales on every marketplace./birthday/01-03is that month and day in every year; a year filter narrows it.- A category filter narrows the listings and sales to one category.
The page counts tracked rare sats only, not every sat mined that day. It also suggests a few days worth a look, such as 3 January 2009 (the genesis block), 9 January 2009 (block 9 and block 1) and 28 November 2012 (the first halving).
Elsewhere
The same mint days drive the date filters of listings and sales: a single date, a month, a year, or a month-day in any year. An item passes when one of its sats was mined on the day asked for. Dates are always UTC.