モバイル開発における日付と時刻：概念、形式、扱い方

import Foundation

// キャッシュされたDateFormatter — 一度だけ作成
private let dateFormatter: DateFormatter = {
    let formatter = DateFormatter()
    formatter.dateFormat = "dd.MM.yyyy HH:mm"
    formatter.locale = Locale(identifier: "ru_RU")
    formatter.timeZone = TimeZone(identifier: "Europe/Moscow")
    return formatter
}()

// ISO8601DateFormatter — 軽量な代替
private let isoFormatter: ISO8601DateFormatter = {
    let formatter = ISO8601DateFormatter()
    formatter.formatOptions = [.withInternetDateTime, .withFractionalSeconds]
    return formatter
}()

let now = Date()
let formattedDate = dateFormatter.string(from: now)
// "01.07.2026 14:30"
let isoDate = isoFormatter.string(from: now)
// "2026-07-01T14:30:00.000Z"

import java.time.LocalDate
import java.time.LocalDateTime
import java.time.ZonedDateTime
import java.time.format.DateTimeFormatter
import java.time.ZoneId

// 現在の日付の作成とフォーマット
val today = LocalDate.now()
val formatter = DateTimeFormatter.ofPattern("dd.MM.yyyy")
val formatted = today.format(formatter)

// タイムゾーンの操作
val utcTime = ZonedDateTime.now(ZoneId.of("UTC"))
val moscowTime = utcTime.withZoneSameInstant(ZoneId.of("Europe/Moscow"))

import Foundation

// UTCからiOSのローカル時刻への変換
func formatToLocalTime(utcDate: Date, timeZoneId: String) -> String {
    let formatter = DateFormatter()
    formatter.dateFormat = "dd.MM.yyyy HH:mm"
    formatter.timeZone = TimeZone(identifier: timeZoneId)
    return formatter.string(from: utcDate)
}

// デバイスのタイムゾーンを特定
let localTimeZone = TimeZone.current
let isDST = localTimeZone.isDaylightSavingTime()
let secondsFromGMT = localTimeZone.secondsFromGMT()

import java.time.Instant
import java.time.ZonedDateTime
import java.time.ZoneId

// Unix Timestampをローカル時刻に変換
fun toLocalDisplay(unixSeconds: Long): String {
    val instant = Instant.ofEpochSecond(unixSeconds)
    val local = instant.atZone(ZoneId.systemDefault())
    return local.format(DateTimeFormatter.ofPattern("dd.MM.yyyy HH:mm"))
}

import org.threeten.bp.LocalDate
import org.threeten.bp.format.DateTimeFormatter

// Applicationでの初期化
class App : Application() {
    override fun onCreate() {
        super.onCreate()
        AndroidThreeTen.init(this)
    }
}

// APIはjava.timeと同一
val today = LocalDate.now()
val formatted = today.format(DateTimeFormatter.ISO_LOCAL_DATE)

import Foundation

// iOSでUnix Timestampを取得
let seconds = Date().timeIntervalSince1970
let milliseconds = Int64(seconds * 1000)

// TrueTimeでNTP同期
TrueTime.shared.start { result in
    switch result {
    case .success:
        let ntpTime = TrueTime.shared.now()
        print("正確な時刻：\(ntpTime)")
    case .failure(let error):
        print("NTPエラー：\(error)")
    }
}