fix Calendar.RecurrenceRule (swiftlang#1284)

Author: lukaskollmer

Sources/FoundationEssentials/Calendar/Calendar_Recurrence.swift

@@ -93,6 +93,9 @@ extension Calendar {
             /// value is used as a lower bound for ``nextBaseRecurrenceDate()``.
             let rangeLowerBound: Date?
 
+            /// The start date's nanoseconds component
+            let startDateNanoseconds: TimeInterval
+            
             /// How many occurrences have been found so far
             var resultsFound = 0
 
@@ -232,6 +235,8 @@ extension Calendar {
                 }
                 var componentsForEnumerating = recurrence.calendar._dateComponents(components, from: start) 
 
+                startDateNanoseconds = start.timeIntervalSince1970.remainder(dividingBy: 1)
+                
                 let expansionChangesDay = dayOfYearAction == .expand || dayOfMonthAction == .expand || weekAction == .expand || weekdayAction == .expand
                 let expansionChangesMonth = dayOfYearAction == .expand || monthAction == .expand || weekAction == .expand
 
@@ -422,6 +427,13 @@ extension Calendar {
                     recurrence._limitTimeComponent(.second, dates: &dates, anchor: anchor)
                 }
 
+                if startDateNanoseconds > 0 {
+                    // `_dates(startingAfter:)` above returns whole-second dates,
+                    // so we need to restore the nanoseconds value present in the original start date.
+                    for idx in dates.indices {
+                        dates[idx] += startDateNanoseconds
+                    }
+                }
                 dates = dates.filter { $0 >= self.start }
 
                 if let limit = recurrence.end.date {

Tests/FoundationEssentialsTests/GregorianCalendarRecurrenceRuleTests.swift

@@ -803,4 +803,16 @@ final class GregorianCalendarRecurrenceRuleTests: XCTestCase {
         ]
         XCTAssertEqual(results, expectedResults)
     }
+    
+    func testDailyRecurrenceRuleWithNonzeroNanosecondComponent() {
+        let start = Date(timeIntervalSince1970: 1746627600.5) // 2025-05-07T07:20:00.500-07:00
+        let rule = Calendar.RecurrenceRule.daily(calendar: gregorian, end: .afterOccurrences(2))
+        let results = Array(rule.recurrences(of: start))
+        
+        let expectedResults: [Date] = [
+            start,
+            Date(timeIntervalSince1970: 1746714000.5), // 2025-05-08T07:20:00.500-07:00
+        ]
+        XCTAssertEqual(results, expectedResults)
+    }
 }