At this moment the DayPilot.Date class supports milliseconds only, i.e. it will accept three digits after the decimal point: "2022-06-01T23:59:59.999".
https://api.daypilot.org/daypilot-date-constructor/
Milliseconds are the maximum precision that is used internally. Parsing a date string with nanoseconds would require rounding which doesn't produce the desired result ("2022-06-02T00:00:00").
However, the library is designed to work with exact time points that are not overlapping. So for a full-day event, you can use "2022-06-01T00:00:00" to "2022-06-02T00:00:00". Such event will not overlap with events starting at "2022-06-02T00:00:00".
For full-day events, you can also use the date part only mode (i.e. both start and end would be "2022-06-01"):
https://doc.daypilot.org/scheduler/event-end-date-time/