Facinating concept. I like how they embed their philosophy into the article by saying the idea was thought up in 01995.
The 10,000 Year Clock extends our perception of time further into the future.
Adopting the Holocene Calendar, as proposed by the geologist Cesare Emiliani in 1993, would extend our understanding of the "long now" further into the past, which I would argue is equally as important.
The best that I can tell is "approximately 11700 years before 1950", which means we're approximately in the year 11768. But approximate years are not enough for people who actually need to type them into calendar apps.
It doesn't even have to be measured from any real event as long as everyone is on the same page. Jesus wasn't born on December 25, 1 AD, either; but nobody cares.