New: CPDate dateByAddingTimeInterval:.

Refs #2126.

Foundation/CPDate.j

@@ -146,6 +146,15 @@ var CPDateReferenceDate = new Date(Date.UTC(2001, 0, 1, 0, 0, 0, 0));
     return [[CPDate date] timeIntervalSinceReferenceDate];
 }
 
+/**
+    Return a new date representing the receiver's time plus the given interval into the future,
+    or into the past for a negative interval.
+*/
+- (id)dateByAddingTimeInterval:(CPTimeInterval)seconds
+{
+    return [[CPDate alloc] initWithTimeInterval:seconds sinceDate:self];
+}
+
 - (BOOL)isEqual:(CPDate)aDate
 {
     if (self === aDate)

Tests/Foundation/CPDateTest.j

@@ -35,6 +35,30 @@
     [self assert:middle equals:[middle laterDate:past] message:"laterDate incorrect"];
 }
 
+- (void)testDateByAddingTimeIntervalZeroShouldReturnSameDate
+{
+    var a = [CPDate dateWithTimeIntervalSince1970:1231889490.0],
+        b = [a dateByAddingTimeInterval:0];
+
+    [self assert:[b timeIntervalSince1970] equals:1231889490];
+}
+
+- (void)testDateByAddingTimeIntervalPositiveShouldReturnNewerDate
+{
+    var a = [CPDate dateWithTimeIntervalSince1970:1231889490.0],
+        b = [a dateByAddingTimeInterval:789];
+
+    [self assert:[b timeIntervalSince1970] equals:1231889490 + 789];
+}
+
+- (void)testDateByAddingTimeIntervalNegativeShouldReturnOlderDate
+{
+    var a = [CPDate dateWithTimeIntervalSince1970:1231889490.0],
+        b = [a dateByAddingTimeInterval:-789];
+
+    [self assert:[b timeIntervalSince1970] equals:1231889490 - 789];
+}
+
 - (void)testEquals
 {
     var now = [CPDate date];