Ответ 1

Сначала создайте NSDate для данного года и месяца:

let dateComponents = NSDateComponents()
dateComponents.year = 2015
dateComponents.month = 7

let calendar = NSCalendar.currentCalendar()
let date = calendar.dateFromComponents(dateComponents)!

Затем используйте метод rangeOfUnit(), как описано в Число дней в текущем месяце с помощью iPhone SDK?:

// Swift 2:
let range = calendar.rangeOfUnit(.Day, inUnit: .Month, forDate: date)
// Swift 1.2:
let range = calendar.rangeOfUnit(.CalendarUnitDay, inUnit: .CalendarUnitMonth, forDate: date)

let numDays = range.length
print(numDays) // 31

Обновление для Swift 3 (Xcode 8):

let dateComponents = DateComponents(year: 2015, month: 7)
let calendar = Calendar.current
let date = calendar.date(from: dateComponents)!

let range = calendar.range(of: .day, in: .month, for: date)!
let numDays = range.count
print(numDays) // 31

Ответ 2

Swift 3.1 и Xcode 8 +

let calendar = Calendar.current
let date = Date()

// Calculate start and end of the current year (or month with `.month`):
let interval = calendar.dateInterval(of: .year, for: date)! //change year it will no of days in a year , change it to month it will give no of days in a current month

// Compute difference in days:
let days = calendar.dateComponents([.day], from: interval.start, to: interval.end).day!
print(days)

Ответ 3

Swift 2.0

func getDaysInMonth(month: Int, year: Int) -> Int
{
    let calendar = NSCalendar.currentCalendar()

    let startComps = NSDateComponents()
    startComps.day = 1
    startComps.month = month
    startComps.year = year

    let endComps = NSDateComponents()
    endComps.day = 1
    endComps.month = month == 12 ? 1 : month + 1
    endComps.year = month == 12 ? year + 1 : year

    let startDate = calendar.dateFromComponents(startComps)!
    let endDate = calendar.dateFromComponents(endComps)!

    let diff = calendar.components(NSCalendarUnit.Day, fromDate: startDate, toDate: endDate, options: NSCalendarOptions.MatchFirst)

    return diff.day
}

let days = getDaysInMonth(4, year: 2015) // April 2015 has 30 days
print(days) // Prints 30

Swift 1.2

func getDaysInMonth(month: Int, year: Int) -> Int
{
    let calendar = NSCalendar.currentCalendar()

    let startComps = NSDateComponents()
    startComps.day = 1
    startComps.month = month
    startComps.year = year

    let endComps = NSDateComponents()
    endComps.day = 1
    endComps.month = month == 12 ? 1 : month + 1
    endComps.year = month == 12 ? year + 1 : year

    let startDate = calendar.dateFromComponents(startComps)!
    let endDate = calendar.dateFromComponents(endComps)!

    let diff = calendar.components(NSCalendarUnit.CalendarUnitDay, fromDate: startDate, toDate: endDate, options: NSCalendarOptions.allZeros)

    return diff.day
}

let days = getDaysInMonth(4, 2015) // There were 30 days in April 2015
println(days) // Prints 30

Ответ 4

В формате расширения используйте self, чтобы иметь возможность динамически возвращать число дней (Swift 3).

extension Date {

func getDaysInMonth() -> Int{
    let calendar = Calendar.current

    let dateComponents = DateComponents(year: calendar.component(.year, from: self), month: calendar.component(.month, from: self))
    let date = calendar.date(from: dateComponents)!

    let range = calendar.range(of: .day, in: .month, for: date)!
    let numDays = range.count

    return numDays
}

}

Ответ 5

Как насчет данного года?

Далее указывается количество дней в месяце:

let range = userCalendar.range(of: .day, in: .year, for: Date())!
print("Days In Year: \(range.count)") // prints 31

... а не количество дней в дате, как ожидалось:/

Ответ 6

Чтобы получить количество дней и все даты по данному месяцу и году, попробуйте это.

func getAllDates(month: Int, year: Int) -> [Date] {
    let dateComponents = DateComponents(year: year, month: month)
    let calendar = Calendar.current
    let date = calendar.date(from: dateComponents)!

    let range = calendar.range(of: .day, in: .month, for: date)!
    let numDays = range.count
    let formatter = DateFormatter()
    formatter.dateFormat = "yyyy MM dd"
    formatter.timeZone = TimeZone(abbreviation: "GMT+0:00")
    var arrDates = [Date]()
    for day in 1...numDays {
        let dateString = "\(year) \(month) \(day)"
        if let date = formatter.date(from: dateString) {
            arrDates.append(date)
        }
    }

    return arrDates
}

Использование:

let arrDatesInGivenMonthYear = getAllDates(month: 1, year: 2018)
debugPrint(arrDatesInGivenMonthYear)
//Output: [2018-01-01 00:00:00 +0000, 2018-01-02 00:00:00 +0000, ... , 2018-01-31 00:00:00 +0000]

let numberOfDays = arrDatesInGivenMonthYear.count
debugPrint(numberOfDays)
//Output: 31