ical-recurrence: test that a date-valued RecurrenceDateTimes does not fail unfixably Unlike DTEND, the value type of RDATE is not constrained to that of DTSTART: RFC 5545 section 3.8.5.2 allows DATE and PERIOD as well as the default DATE-TIME. Combined with a DTEND, a date-valued RDATE reaches computeNewEnd with a DateTimeStartDate as the new start and a DateTimeStartDateTime as the original one, which is an unfixable StartStartMismatch. That error is documented in ICal.Recurrence.Class as an internal error that should not happen, yet it is reachable from ordinary parsed input and it halts recurrence for the whole calendar. Reading the added instance as an all-day one is what its value type says. Coercing it to a time on that day would be defensible too; failing the whole calendar is not.