Date and time widgets
Every date, range, duration and recurrence widget installed on this site, built from the real field widget plugins rather than rebuilt by hand -- a date widget is rarely one element, and copying it would be copying the thing under test. Core's date, range, datelist and timestamp widgets; the recurring ones from Date Recur and its four modular rewrites of the same rule; Smart Date, which thinks in durations rather than end times; Duration Field and Interval, which store a length with no start; and the pickers from Flatpickr, Duet, Single Date Time and Datetime Range Popup. The field definitions are made up on the spot, so no fields need to exist and nothing is saved. A widget whose module is not installed is left out rather than shown broken.