【Swift】Calendar構造体の使い方！暦法の変更と日付の計算

let calendar = Calendar.current

let calendar = Calendar(identifier: .gregorian)

let calendar = Calendar(identifier: .gregorian)
var dateComponent = DateComponents(calendar: Calendar.current, year: 2022, month: 10, day: 4)
let date = calendar.date(from: dateComponent)
print(date)
// Optional(2022-10-03 15:00:00 +0000)

let calendar = Calendar(identifier: .gregorian)
var dateComponent = calendar.dateComponents(in: TimeZone.current, from: Date())
print(dateComponent)
// 結果：calendar: gregorian (fixed) timeZone: Asia/Tokyo (fixed (equal to current)) era: 1 year: 2023 month: 4 day: 18 hour: 19 minute: 17 second: 8 nanosecond: 276412963 weekday: 3 weekdayOrdinal: 3 quarter: 0 weekOfMonth: 4 weekOfYear: 16 yearForWeekOfYear: 2023 isLeapMonth: false

llet calendar = Calendar(identifier: .gregorian)
let now = Date() // 現在日時インスタンスを生成
// 時刻を初期化したインスタンスを生成
let startToday = calendar.startOfDay(for: now)

print(startToday)
// 2023-04-17 15:00:00 +0000
print(startToday.description(with: Locale(identifier: "ja_JP")))
// 2023年4月18日 火曜日 0時00分00秒 日本標準時

func component(
    _ component: Calendar.Component,
    from date: Date
) -> Int

let calendar = Calendar.current
let now = Date() // "Apr 18, 2023 at 7:31 PM"
let day = calendar.component(.day, from: now)
print(day) // 18

let calendar = Calendar.current
let now = Date() // "Apr 18, 2023 at 7:29 PM"
let modifiedDate = calendar.date(byAdding: .day,value: 3,to: now)
print(modifiedDate!.description(with: Locale(identifier: "ja_JP")))
// 2023年4月21日 金曜日 19時29分14秒 日本標準時

let calendar = Calendar.current
let startDate = calendar.date(from: DateComponents(year: 2023, month: 4, day: 1))!
let endDate = calendar.date(from: DateComponents(year: 2023, month: 4, day: 30))!

let dateInterval = DateInterval(start: startDate, end: endDate)
let numberOfDays = calendar.dateComponents([.day], from: dateInterval.start, to: dateInterval.end).day

print("日数: \(numberOfDays ?? 0)")
// 日数: 29

func dateInterval(
    of component: Calendar.Component,
    for date: Date
) -> DateInterval?

let calendar = Calendar.current

let weekInterval = calendar.dateInterval(of: .weekOfMonth, for:.now)
print(weekInterval!)
// 2023-04-15 15:00:00 +0000 to 2023-04-22 15:00:00 +0000

let calendar = Calendar.current
let now = Date()
let initSecondsday = calendar.date(bySetting: .second, value: 0, of: now)
print(initSecondsday) // Optional(2024-02-05 11:40:00 +0000)