The calculator runs in your browser. The dates, numbers and days off you enter are not sent to any server; they are kept in the page address so you can bookmark or share an answer, and only you decide who sees that link.
The site does not use analytics, advertising or tracking cookies. Fonts are loaded from Google Fonts, which receives the usual technical request data (such as your IP address) when your browser fetches them. If anything is added later, this page will say so first.