The toStringSortable method returns the current DayPilot.Date value as a sortable ISO 8601 string.
DayPilot.Date.toStringSortable()string - ISO 8601 date/time string suitable for lexicographic sorting.
The returned string uses the sortable ISO 8601 representation of the current value.
const date = new DayPilot.Date("2020-12-31T13:00:00");
const value = date.toStringSortable();