日付 — 基礎、タイムゾーン、Intl.DateTimeFormat

日付の導入

Date値の作成

// Create Date instances
// Now (current time)
const now = new Date();

// From timestamp (milliseconds since Jan 1, 1970 UTC)
const fromMs = new Date(0);

// From parts: year, monthIndex (0=Jan), day
const xmas = new Date(2025, 11, 25);

console.log("Now:", now.toString());
console.log("Epoch:", fromMs.toUTCString());
console.log("Xmas:", xmas.toDateString());

各部分のgetter

// Read parts with getters
const d = new Date(2025, 0, 2, 15, 30);

console.log("Year:", d.getFullYear());
console.log("Month (0=Jan):", d.getMonth());
console.log("Day of month:", d.getDate());
console.log("Hours:", d.getHours());
console.log("Minutes:", d.getMinutes());

ISO形式とエポック

// ISO string and timestamps
const sample = new Date();

console.log("ISO:", sample.toISOString());
console.log("Unix seconds (approx):", Math.floor(sample.getTime() / 1000));

ロケールに応じた整形

// Format for a locale
const date = new Date(2025, 6, 4, 9, 5);

// Simple: toLocaleString uses your environment defaults
console.log("Local:", date.toLocaleString());

// Intl.DateTimeFormat with explicit locale and options
const fmt = new Intl.DateTimeFormat("en-GB", {
  year: "numeric",
  month: "short",
  day: "2-digit",
  hour: "2-digit",
  minute: "2-digit"
});

console.log("Formatted:", fmt.format(date));

別のタイムゾーンで整形

// Format for a different time zone
const meeting = new Date(Date.UTC(2025, 3, 10, 12, 0));

const ny = new Intl.DateTimeFormat("en-US", {
  timeZone: "America/New_York",
  dateStyle: "medium",
  timeStyle: "short"
});

const tokyo = new Intl.DateTimeFormat("en-US", {
  timeZone: "Asia/Tokyo",
  dateStyle: "medium",
  timeStyle: "short"
});

console.log("NY:", ny.format(meeting));
console.log("Tokyo:", tokyo.format(meeting));

Intl.DateTimeFormatのクイズ

まとめ