Dates around DST end

Dates.swift

import Foundation

let calendar = Calendar(identifier: .gregorian)



// Daylight Saving Time ends in 2017 on Nov 5th, 2:00 AM in the United States.
// Let's instantiate some `Date`s around that.
let nov4_1am = calendar.date(from: DateComponents(year: 2017, month: 11, day: 4, hour: 1))!
let nov4_2am = calendar.date(from: DateComponents(year: 2017, month: 11, day: 4, hour: 2))!
let nov4_3am = calendar.date(from: DateComponents(year: 2017, month: 11, day: 4, hour: 3))!

let nov5_1am = calendar.date(from: DateComponents(year: 2017, month: 11, day: 5, hour: 1))!
let nov5_2am = calendar.date(from: DateComponents(year: 2017, month: 11, day: 5, hour: 2))!
let nov5_3am = calendar.date(from: DateComponents(year: 2017, month: 11, day: 5, hour: 3))!

// We should always rely on a calendar to get specific dates. Assuming that
// a day always has 86400s (24*60*60) can break, as we can see below.
Date(timeIntervalSince1970: nov4_1am.timeIntervalSince1970 + 86400) // That's expected…
Date(timeIntervalSince1970: nov4_2am.timeIntervalSince1970 + 86400) // … but this?
Date(timeIntervalSince1970: nov5_1am.timeIntervalSince1970 + 3600)  // Or this?

// We cannot rely on this assumption for calculating the difference between two dates.
func naiveDaysDifference(from: Date, to: Date) -> Int {
    let timestampDifference = to.timeIntervalSince1970 - from.timeIntervalSince1970
    return Int((timestampDifference / 86400).rounded(.up))
}

// We can and should use the Foundation classes instead though!
func calendarBasedDaysDifference(from: Date, to: Date) -> Int {
    return calendar.dateComponents(Set([Calendar.Component.day]), from: from, to: to).day!
}

// Helper function
func compare(from: Date, to: Date) -> (naive: Int, calendarBased: Int) {
    return (naiveDaysDifference(from: from, to: to), calendarBasedDaysDifference(from: from, to: to))
}

compare(from: nov4_1am, to: nov5_1am)
compare(from: nov4_2am, to: nov5_2am) // 2 ≠ 1 ⚡️
compare(from: nov4_3am, to: nov5_3am)