See http://mmertsock.github.io/2015/01/23/undocumented-nscalendar/ for more info.

NSCalendarTests.swift

import XCTest

func NSCalendarForTesting() -> NSCalendar {
    let calendar = NSCalendar(calendarIdentifier: NSGregorianCalendar)!
    calendar.timeZone = NSTimeZone(name: "America/New_York")!
    calendar.firstWeekday = 2 // Monday
    return calendar
}

class NSCalendarTests: XCTestCase {
    
    let calendar = NSCalendarForTesting()
    var date20140703noon: NSDate!
    var baseComps: NSDateComponents!
    
    override func setUp() {
        super.setUp()
        baseComps = NSDateComponents()
        baseComps.year = 2014
        baseComps.month = 7
        baseComps.day = 3
        baseComps.hour = 12
        baseComps.minute = 0
        baseComps.second = 0
        date20140703noon = calendar.dateFromComponents(baseComps)!
    }

    func testStartOfDayForDate() {
        let newDate = calendar.startOfDayForDate(date20140703noon)
        let comps = comparisonComponents(newDate)
        XCTAssertEqual(comps.year, baseComps.year, "same year")
        XCTAssertEqual(comps.month, baseComps.month, "same month")
        XCTAssertEqual(comps.day, baseComps.day, "same day")
        XCTAssertEqual(comps.hour, 0, "midnight (hour)")
        XCTAssertEqual(comps.minute, 0, "midnight (minutes)")
        XCTAssertEqual(comps.second, 0, "midnight (seconds)")
    }
    
    func testStartOfDayForDateEdgeCases() {
        let c = NSDateComponents()
        c.year = 2014
        c.month = 12
        c.day = 31
        c.hour = 23
        c.minute = 59
        c.second = 59
        let baseDate = calendar.dateFromComponents(c)!
        var tomorrow = calendar.startOfNextDayForDate(baseDate)
        var comps = comparisonComponents(tomorrow)
        XCTAssertEqual(comps.year, c.year + 1, "Next year")
        XCTAssertEqual(comps.month, 1, "January")
        XCTAssertEqual(comps.day, 1, "First")
        XCTAssertEqual(comps.hour, 0, "Midnight (hour)")
        XCTAssertEqual(comps.minute, 0, "Midnight (minute)")

        // DST: calendar day is less than 24 hours
        
        c.year = 2015
        c.month = 3
        c.day = 8
        c.hour = 12
        c.minute = 0
        c.second = 0
        let noonOnDSTChangeDay = calendar.dateFromComponents(c)!
        let morningOnDSTChangeDay = calendar.startOfDayForDate(noonOnDSTChangeDay)
        comps = comparisonComponents(morningOnDSTChangeDay)
        XCTAssertEqual(comps.year, 2015, "Same year")
        XCTAssertEqual(comps.month, 3, "Same month")
        XCTAssertEqual(comps.day, 8, "Same day")
        XCTAssertEqual(comps.hour, 0, "Midnight (hour)")
        XCTAssertEqual(comps.minute, 0, "Midnight (minute)")
        
        c.hour = 1
        let justBeforeDSTChange = calendar.dateFromComponents(c)!
        tomorrow = calendar.startOfNextDayForDate(justBeforeDSTChange)
        comps = comparisonComponents(tomorrow)
        XCTAssertEqual(comps.year, 2015, "Same year")
        XCTAssertEqual(comps.month, 3, "Same month")
        XCTAssertEqual(comps.day, 9, "Next day")
        XCTAssertEqual(comps.hour, 0, "Midnight (hour)")
        XCTAssertEqual(comps.minute, 0, "Midnight (minute)")
    }

    private func comparisonComponents(date: NSDate) -> NSDateComponents {
        return calendar.components(.CalendarUnitYear | .CalendarUnitMonth | .CalendarUnitDay | .CalendarUnitHour | .CalendarUnitMinute | .CalendarUnitSecond, fromDate: date)
    }
}