Thanks for sharing that!
“This cookie tracks anonymous visitor website activity statistics.” (source)
We do not have any tracking cookies in our calendar plugins.
My hunch is it comes from the Slimstat Analytics plugin. If you have that plugin installed and active, please deactivate it, clear your cache and cookies, and check if it gets re-set.
If yes, then please go through our conflict testing guide, might help you discover where the cookie is coming from.
Also please be so kind and share your system information with me, so I can check if there is anything out of line in terms of our plugins.
Thanks and cheers,
Andras