RecurrenceRule produces incorrect sequences for start dates with non-zero nanosecond components

[lukaskollmer]

SF-0009 states that the first element in a sequence of Date occurrences computed from a RecurrenceRule is always the start date (see SF-0009#Usage):

A recurrence rule of a given frequency repeats the start date with the interval of that frequency. For example, assume that now it is February 09 2024, 13:43. Creating a daily recurrence would yield a result for each following date at the same time:

var recurrence = Calendar.RecurrenceRule(calendar: .current, frequency: .daily)
for date in recurrence.recurrences(of: .now) {
    // 2024-02-09, 13:43
    // 2024-02-10, 13:43
    // 2024-02-11, 13:43
    // 2024-02-12, 13:43
    // ...
}

However, this currently is only the case for start dates with a 0 nanosecond component. Any other start date (e.g. also Date.now, unless you happen to be creating the date exactly at the start of a second, which admittedly is rather unlikely), will result in recurrences(of:) returning a sequence that will skip the start date.

The reason for this is that Calendar.DatesByRecurring.Iterator internally uses Calendar._dates(startingAfter:), which in turn uses Calendar._unadjustedDates(after:), which only produces Dates with whole-integer seconds.

The tests currently don't catch this, since all RecurrenceRule tests only use dates with whole-integer components as input.