Using NSDate to get date for Easter - swift

Question

I'm working on an application that requires the use of getting dates for national holidays.
Below, I was able to get Memorial Day:
// Set the components for Memorial Day (last Monday of May)
let memorialDayComps = NSDateComponents()
memorialDayComps.weekday = 2
memorialDayComps.month = 5
memorialDayComps.year = currentYear
var mondaysOfMay = [NSDate]()
for var i = 1; i <= 5; i++ {
memorialDayComps.weekdayOrdinal = i
let monday = calendar.dateFromComponents(memorialDayComps)
let components = calendar.components(.CalendarUnitMonth, fromDate: monday!)
if components.month == 5 {
mondaysOfMay.append(monday!)
}
}
let memorialDayDate = mondaysOfMay.last
Because the dates are pretty well set, I am able to successfully create NSDate instances for the following holidays:
New Year's Day
Martin Luther King, Jr. Day
Presidents' Day
Memorial Day
Independence Day
Labor Day
Thanksgiving Day
Christmas Day
However, the only one that I am having difficulty figuring out how to get is Easter. It varies every year, so I'm curious as to whether anyone else has been able so successfully get the date for Easter via an API or other means.

Answer 1

I was able to find a gist on GitHub that has a solution that was accurate for calculating and returning an NSDate for Easter.
The code below is what the gist contains:
// Easter calculation in swift after Anonymous Gregorian algorithm
// Also known as Meeus/Jones/Butcher algorithm
func easter(Y : Int) -> NSDate {
let a = Y % 19
let b = Int(floor(Double(Y) / 100))
let c = Y % 100
let d = Int(floor(Double(b) / 4))
let e = b % 4
let f = Int(floor(Double(b+8) / 25))
let g = Int(floor(Double(b-f+1) / 3))
let h = (19*a + b - d - g + 15) % 30
let i = Int(floor(Double(c) / 4))
let k = c % 4
let L = (32 + 2*e + 2*i - h - k) % 7
let m = Int(floor(Double(a + 11*h + 22*L) / 451))
let components = NSDateComponents()
components.year = Y
components.month = Int(floor(Double(h + L - 7*m + 114) / 31))
components.day = ((h + L - 7*m + 114) % 31) + 1
components.timeZone = NSTimeZone(forSecondsFromGMT: 0)
let cal = NSCalendar(calendarIdentifier: NSGregorianCalendar)
return cal.dateFromComponents(components)
}
println(easter(2014)) // "2014-04-20 00:00:00 +0000"

Answer 2

That Easter algorithm works great!
Using with Swift 4.0 and pattern matching. Pattern matching made it easier for me to add other days based on month, day, weekday, weekdayOrdinal.
extension Date {
var isUSHoliday: Bool {
let components = Calendar.current.dateComponents([.year, .month, .day, .weekday, .weekdayOrdinal], from: self)
guard let year = components.year,
let month = components.month,
let day = components.day,
let weekday = components.weekday,
let weekdayOrdinal = components.weekdayOrdinal else { return false }
let easterDateComponents = Date.dateComponentsForEaster(year: year)
let easterMonth: Int = easterDateComponents?.month ?? -1
let easterDay: Int = easterDateComponents?.day ?? -1
let memorialDay = Date.dateComponentsForMemorialDay(year: year)?.day ?? -1
// weekday is Sunday==1 ... Saturday==7
// weekdayOrdinal is nth instance of weekday in month
switch (month, day, weekday, weekdayOrdinal) {
case (1, 1, _, _): return true // Happy New Years
case (1, 0, 2, 3): return true // MLK - 3rd Mon in Jan
case (2, 0, 2, 3): return true // Washington - 3rd Mon in Feb
case (easterMonth, easterDay, _, _): return true // Easter - rocket science calculation
case (5, memorialDay, _, _): return true // Memorial Day
case (7, 4, _, _): return true // Independence Day
case (9, 0, 2, 1): return true // Labor Day - 1st Mon in Sept
case (10, 0, 2, 2): return true // Columbus Day - 2nd Mon in Oct
case (11, 11, _, _): return true // Veterans Day
case (11, 0, 5, 4): return true // Happy Thanksgiving - 4th Thurs in Nov
case (12, 25, _, _): return true // Happy Holidays
case (12, 31, _, _): return true // New years Eve
default: return false
}
}
static func dateComponentsForMemorialDay(year: Int) -> DateComponents? {
guard let memorialDay = Date.memorialDay(year: year) else { return nil }
return NSCalendar.current.dateComponents([.year, .month, .day, .weekday, .weekdayOrdinal], from: memorialDay)
}
static func memorialDay(year: Int) -> Date? {
let calendar = Calendar.current
var firstMondayJune = DateComponents()
firstMondayJune.month = 6
firstMondayJune.weekdayOrdinal = 1 // 1st in month
firstMondayJune.weekday = 2 // Monday
firstMondayJune.year = year
guard let refDate = calendar.date(from: firstMondayJune) else { return nil }
var timeMachine = DateComponents()
timeMachine.weekOfMonth = -1
return calendar.date(byAdding: timeMachine, to: refDate)
}
static func easterHoliday(year: Int) -> Date? {
guard let dateComponents = Date.dateComponentsForEaster(year: year) else { return nil }
return Calendar.current.date(from: dateComponents)
}
static func dateComponentsForEaster(year: Int) -> DateComponents? {
// Easter calculation from Anonymous Gregorian algorithm
// AKA Meeus/Jones/Butcher algorithm
let a = year % 19
let b = Int(floor(Double(year) / 100))
let c = year % 100
let d = Int(floor(Double(b) / 4))
let e = b % 4
let f = Int(floor(Double(b+8) / 25))
let g = Int(floor(Double(b-f+1) / 3))
let h = (19*a + b - d - g + 15) % 30
let i = Int(floor(Double(c) / 4))
let k = c % 4
let L = (32 + 2*e + 2*i - h - k) % 7
let m = Int(floor(Double(a + 11*h + 22*L) / 451))
var dateComponents = DateComponents()
dateComponents.month = Int(floor(Double(h + L - 7*m + 114) / 31))
dateComponents.day = ((h + L - 7*m + 114) % 31) + 1
dateComponents.year = year
guard let easter = Calendar.current.date(from: dateComponents) else { return nil } // Convert to calculate weekday, weekdayOrdinal
return Calendar.current.dateComponents([.year, .month, .day, .weekday, .weekdayOrdinal], from: easter)
}
}

Answer 3

Here is a Swift 5 implementation of O'Beirne's algorithm with inline documentation.
The code is more compact than the implementations in the other provided answers because it makes use of Integer arithmetic and thus removes the need to explicitly round numbers and convert between Floats and Ints.
/// **How ten divisions lead to Easter** *by T. H. O'Beirne, New Scientist, march 30 1961 - Vol. 9,Nr. 228*
func easter(in year: Int) -> (day: Int, month: Int) {
/// Identify the position of the `year` in a 19-year cycle, to use this later to determine the principal constituent of the changes of full-moon dates from year to year
let a = year % 19
/// Take note of the corrections which the Gregorian calendar introduces in century years
let (b, c) = year.quotientAndRemainder(dividingBy: 100)
/// Take account of the leap-year exceptions in century years
let (d, e) = b.quotientAndRemainder(dividingBy: 4)
/// Provide similarly for the century years auxiliary corrections to the new-moon and full-moon dates
let g = (8*b + 13) / 25
/// Determine the number of days between 21 March and the coincident or next full moon, if no special exceptions arise
let h = (19*a + b - d - g + 15) % 30
/// Determine the position of the year in the ordinary leap-year cycle of four years
let (i, k) = c.quotientAndRemainder(dividingBy: 4)
/// Determine number of days (between 0 and 6) until the Sunday *after* full moon
let l = (2*e + 2*i - h - k + 32) % 7
/// The exceptions which make a 29-day month interrupt the regularity of a simpler pattern need here be considered *only* when they transfer the full moon *from a Sunday to a Saturday*: the *Easter date* is unaffected in other cases. When appropriate — 1954 and 1981 are quite rare examples — we have m=1; otherwise m=0 : this permits the necessary correction (failing which the Easter date *would* be 26 April in 1981.
let m = (a + 11*h + 19*l) / 433
/// Determine days between March 22 and Easter
let relativeDayCount = h + l - 7*m
/// Convert relative day count into absolute month and day index
let month = (relativeDayCount + 90) / 25
return (day: (relativeDayCount + 33*month + 19) % 32, month)
}
func easterDate(in year: Int) -> Date {
let (day, month) = easter(in: year)
let components = DateComponents(
timeZone: TimeZone(secondsFromGMT: 0),
year: year, month: month, day: day
)
return Calendar(identifier: .gregorian).date(from: components)!
}

Answer 4

Swift 4:
func easter(Y : Int) -> Date {
let a = Y % 19
let b = Int(floor(Double(Y) / 100))
let c = Y % 100
let d = Int(floor(Double(b) / 4))
let e = b % 4
let f = Int(floor(Double(b+8) / 25))
let g = Int(floor(Double(b-f+1) / 3))
let h = (19*a + b - d - g + 15) % 30
let i = Int(floor(Double(c) / 4))
let k = c % 4
let L = (32 + 2*e + 2*i - h - k) % 7
let m = Int(floor(Double(a + 11*h + 22*L) / 451))
var components = DateComponents()
components.year = Y
components.month = Int(floor(Double(h + L - 7*m + 114) / 31))
components.day = ((h + L - 7*m + 114) % 31) + 1
components.timeZone = TimeZone(secondsFromGMT: 0)
return Calendar.autoupdatingCurrent.date(from: components)!
}
print(easter(Y: 2018)) // "2018-04-01 00:00:00 +0000"

Answer 5

OBJECTIVE-C!
-(void) easterMonthAndDayForYear: (NSInteger) Y {
NSInteger a = Y % 19;
NSInteger b = (int) (floor( ((double)Y) / 100.0));
NSInteger c = Y % 100;
NSInteger d = (int)(floor(((double)b) / 4.0));
NSInteger e = b % 4;
NSInteger f = (int)(floor(((double)(b+8)) / 25.0));
NSInteger g = (int)(floor(((double)(b-f+1)) / 3.0));
NSInteger h = (19*a + b - d - g + 15) % 30;
NSInteger i = (int)(floor(((double)c) / 4.0));
NSInteger k = c % 4;
NSInteger L = (32 + 2*e + 2*i - h - k) % 7;
NSInteger m = (int)(floor(((double)(a + 11*h + 22*L)) / 451.0));
NSDateComponents *components = [[NSCalendar currentCalendar] components:NSCalendarUnitDay | NSCalendarUnitMonth | NSCalendarUnitYear |NSCalendarUnitTimeZone fromDate:[NSDate date]];
components.year = Y;
components.month = (int)(floor((double)(h + L - 7*m + 114) / 31.0));
components.day = ((h + L - 7*m + 114) % 31) + 1;
components.timeZone = [NSTimeZone timeZoneForSecondsFromGMT:0];
self.easterDayCache[#(Y)] = #{#"month": #(components.month), #"day":#(components.day)
};