Fix NSCalendar date(byAdding:to:options:) improperly returning nil

date(byAdding:to:options:) was always trying to add the components to CFAbsoluteTime 0.0, rather than the passed-in date.

Author: Jason Ji

Foundation/NSCalendar.swift

@@ -579,7 +579,7 @@ open class NSCalendar : NSObject, NSCopying, NSSecureCoding {
 
     open func date(byAdding comps: DateComponents, to date: Date, options opts: Options = []) -> Date? {
         var (vector, compDesc) = _convert(comps)
-        var at: CFAbsoluteTime = 0.0
+        var at: CFAbsoluteTime = date.timeIntervalSinceReferenceDate
 
         let res: Bool = withUnsafeMutablePointer(to: &at) { t in
             return vector.withUnsafeMutableBufferPointer { (vectorBuffer: inout UnsafeMutableBufferPointer<Int32>) in

TestFoundation/TestNSCalendar.swift

@@ -24,6 +24,7 @@ class TestNSCalendar: XCTestCase {
             ("test_gettingDatesOnHebrewCalendar", test_gettingDatesOnHebrewCalendar ),
             ("test_gettingDatesOnChineseCalendar", test_gettingDatesOnChineseCalendar),
             ("test_copy",test_copy),
+            ("test_addingDates", test_addingDates)
             // Disabled because this fails on linux https://bugs.swift.org/browse/SR-320
             // ("test_currentCalendarRRstability", test_currentCalendarRRstability),
         ]
@@ -89,6 +90,17 @@ class TestNSCalendar: XCTestCase {
         XCTAssertEqual(copy.firstWeekday, 2)
         XCTAssertEqual(copy.minimumDaysInFirstWeek, 2)
     }
+    
+    func test_addingDates() {
+        var calendar = Calendar.current
+        let today = calendar.current.date(from: DateComponents(year: 2016, month: 10, day: 4))
+        let diffComponents = DateComponents(day: 1)
+        let tomorrow = calendar.date(byAdding: diffComponents, to: today)
+        
+        XCTAssertEqual(tomorrow.year, 2016)
+        XCTAssertEqual(tomorrow.month, 10)
+        XCTAssertEqual(tomorrow.day, 5)
+    }
 }
 
 class TestNSDateComponents: XCTestCase {