A lot of times, developers will want to show the date selected to the end-user in one format but send the date in the form submission (to a server) in another format. In the published version of this website, you'll see that once the user selects their date, it appears in the format of MONTH DD, YYYY. (Example: October 3, 2019)
However, the value that form actually receives is 2019-10-03. Flatpickr.js allows for a WIDE variety of formats, all of which you can find here.