It was some time ago that I read of that, so the details are fuzzy. And here’s what I found:

https://nypost.com/2019/07/25/10000-year-old-engraved-stone-could-be-worlds-oldest-lunar-calendar/

“A 10,000-year-old engraved stone could be a lunar calendar. The rare pebble — found high up in the mountains near Rome, Italy, the hammer-stone was found on top of Monte Alta in the Alban Hills. It’s believed that our early ancestors would’ve used the stone to keep track of the moon’s cycles. Notches were engraved “as if they were being used to count, calculate or store the record of some kind of information. And these notches — which total either 27 or 28 — suggest the stone’s engraver used the pebble to track lunar cycles.” ref

It could also have been The Ishango Bone (https://www.thedailybeast.com/the-ishango-bone-the-worlds-oldest-period-tracker/)