Swift-从一年中的日期获取日期

Question

We can get day of year for date using below line.

let day = cal.ordinalityOfUnit(.Day, inUnit: .Year, forDate: date)

But how can we get the date from day of year?

Answer 1

If you know the year you can get DateComponents date property as follow:

extension Calendar {
    static let iso8601 = Calendar(identifier: .iso8601)
}


let now = Date()
let day  = Calendar.iso8601.ordinality(of: .day, in: .year, for: now)!  // 121
let year = Calendar.iso8601.component(.year, from: now)  // 2017
let date = DateComponents(calendar: .iso8601, year: year, day: day).date   //  "May 1, 2017, 12:00 AM"

or using DateFormatter

let dateFormatter = DateFormatter()
dateFormatter.dateFormat = "yyyy D"
if let date = dateFormatter.date(from: "\(year) \(day)") {
    dateFormatter.dateStyle = .medium
    dateFormatter.timeStyle = .short
    dateFormatter.string(from: date)    // "May 1, 2017, 12:00 AM"
}

Answer 2

You cannot go the other way. Going from a date to a day of the year discards all other information, you are left with only the day of the year (you no longer know what year). To go back to a full date you would have to make assumptions about the year the day was in.

The answer that @LeoDabus gave is more succinct than this, so it is perhaps the better choice. Having said that, this is the code that I would have used:

let dateComponents = NSDateComponents();
dateComponents.year = 2015
dateComponents.day = day
let calendar = NSCalendar.currentCalendar()
let date = calendar.dateFromComponents(dateComponents)

Answer 3

Updated for Swift 4:

let dateComponents = NSDateComponents();
dateComponents.year = 2018
dateComponents.day = someDay
let calendar = NSCalendar.current
let date = calendar.date(from: dateComponents as DateComponents)