[5.3] Date: Add distance(to:) and advanced(by:) introduced in macOS 10.15

Sources/Foundation/Date.swift

@@ -190,7 +190,22 @@ public struct Date : ReferenceConvertible, Comparable, Equatable {
     public static func -=(lhs: inout Date, rhs: TimeInterval) {
         lhs = lhs - rhs
     }
-
+
+    public typealias Stride = TimeInterval
+
+    /// Returns the `TimeInterval` between this `Date` and another given date.
+    ///
+    /// - returns: The interval between the receiver and the another parameter. If the receiver is earlier than `other`, the return value is negative.
+    public func distance(to other: Date) -> TimeInterval {
+        return other.timeIntervalSince(self)
+    }
+
+    /// Creates a new date value by adding a `TimeInterval` to this `Date`.
+    ///
+    /// - warning: This only adjusts an absolute value. If you wish to add calendrical concepts like hours, days, months then you must use a `Calendar`. That will take into account complexities like daylight saving time, months with different numbers of days, and more.
+    public func advanced(by n: TimeInterval) -> Date {
+        return self.addingTimeInterval(n)
+    }
 }
 
 extension Date : CustomDebugStringConvertible, CustomStringConvertible, CustomReflectable {

Tests/Foundation/Tests/TestDate.swift

@@ -34,6 +34,8 @@ class TestDate : XCTestCase {
             ("test_timeIntervalSinceReferenceDate", test_timeIntervalSinceReferenceDate),
             ("test_recreateDateComponentsFromDate", test_recreateDateComponentsFromDate),
             ("test_Hashing", test_Hashing),
+            ("test_advancedBy", test_advancedBy),
+            ("test_distanceTo", test_distanceTo),
         ]
     }
 
@@ -172,4 +174,22 @@ class TestDate : XCTestCase {
         ]
         checkHashable(values, equalityOracle: { $0 == $1 })
     }
+
+    func test_advancedBy() {
+        let date1 = dateWithString("2010-05-17 14:49:47 -0000")
+        let date2 = dateWithString("2010-05-18 14:49:47 -0000")
+
+        XCTAssertEqual(date1.advanced(by: 86400), date2)
+        XCTAssertEqual(date2.advanced(by: -86400), date1)
+        XCTAssertEqual(date1.advanced(by: 0), date1)
+    }
+
+    func test_distanceTo() {
+        let date1 = dateWithString("2010-05-17 14:49:47 -0000")
+        let date2 = dateWithString("2010-05-18 14:49:47 -0000")
+
+        XCTAssertEqual(date1.distance(to: date2), 86400)
+        XCTAssertEqual(date2.distance(to: date1), -86400)
+        XCTAssertEqual(date1.distance(to: date1), 0)
+    }
 }