Issue Description:

The Date referenceDate is 2001.01.01 00:00 UTC. However if you create a DateFormatter without a year, the effective referenceDate is 2000.01.01 00:00 UTC.

Given that the Date object allows for timeIntervals from both 1970 and 2001 it would more consistent if the DateFormatter used one of the expected epochs instead of using it's own.

To replicate;

import Foundation{{ }}

let d = DateFormatter()

d.dateFormat = "mm:ss"
d.timeZone = TimeZone(abbreviation: "UTC"){{ }}

let date = d.date(from: "10:00")

let timeInterval = date?.timeIntervalSinceReferenceDate{{ }}

let zero = Date(timeIntervalSinceReferenceDate: 0)

print ("(zero)")