Swiftで1年の日数を数える #Swift

実行環境、Xcode 9.2 + Swift 4.0

let calendar = Calendar(identifier: .gregorian)
for year in 2010...2020 {
    var components = DateComponents()
    components.year = year
    components.month = 1
    components.day = 1
    let date1 = calendar.date(from: components)!

    components.year = year + 1
    components.month = 1
    components.day = 1
    let date2 = calendar.date(from: components)!

    components = calendar.dateComponents([.day], from: date1, to: date2)
    print("\(year)年 \(components.day!)日")
}

結果。うるう年は366日になっています。