Migration from Day.js to Temporal API

temporal.ts

/**
 * date string
 */
const s  = "20260430";
const d1 = Temporal.PlainDate.from(s);  // 2026-04-30
const d2 = Temporal.Now.plainDateISO(); // 2026-05-01

// .isBefore() / .isAfter()
Temporal.PlainDate.compare(d1, d2); // -1
Temporal.PlainDate.compare(d1, d1); //  0
Temporal.PlainDate.compare(d2, d1); //  1


/**
 * .startOf() / .endOf()
 */
const t = Temporal.Now.zonedDateTimeISO();

// start of Day
t.startOfDay();
// start of Month
t.with({ day: 1 }).startOfDay();
// start of Year
t.with({ day: 1, month: 1 }).startOfDay();

// end of Day
t.add({ days: 1 }).startOfDay().subtract({ nanoseconds: 1 });
// end of Month
t.add({ months: 1 }).with({ day: 1 }).startOfDay().subtract({ nanoseconds: 1 });
// end of Year
t.add({ years: 1 }).with({ day: 1, month: 1 }).startOfDay().subtract({ nanoseconds: 1 });


/**
 * TimeZone
 */
t.withTimeZone("Asia/Tokyo");


/**
 * Type Conversions
 */

// Instant -> Date
new Date(Temporal.Now.instant().epochMilliseconds);
// ZonedDateTime -> Date
new Date(Temporal.Now.zonedDateTimeISO().epochMilliseconds);

// Date -> Instant
new Date().toTemporalInstant();
// Date -> ZonedDateTime
new Date().toTemporalInstant().toZonedDateTimeISO(Temporal.Now.timeZoneId());


/**
 * Format
 * - Intl.DateTimeFormat doesn't care about TimeZone
 */

// "YYYY-MM-DD"
Intl.DateTimeFormat("en-CA", { dateStyle: "short" }).format(dateLike);

// "YYYY-MM-DD HH:mm"
Intl.DateTimeFormat("en-CA", {
  year: "numeric", month: "2-digit", day: "2-digit",
  hour: "2-digit", minute: "2-digit", hour12: false,
}).format(dateLike).replace(",", "");

// "YYYY/MM/DD"
Intl.DateTimeFormat("ja-JP", { dateStyle: "medium" }).format(dateLike);
Intl.DateTimeFormat("ja-JP", { dateStyle: "short"  }).format(dateLike);

// "YYYY/MM/DD HH:mm"
Intl.DateTimeFormat("ja-JP", { dateStyle: "medium", timeStyle: "short" }).format(dateLike);
Intl.DateTimeFormat("ja-JP", { dateStyle: "short",  timeStyle: "short" }).format(dateLike);

// "MM/DD"
Intl.DateTimeFormat("ja-JP", { month: "2-digit", day: "2-digit" }).format(dateLike);

// "HH:mm:ss"
Intl.DateTimeFormat("ja-JP", { timeStyle: "medium" }).format(dateLike);

// "HH:mm"
Intl.DateTimeFormat("ja-JP", { timeStyle: "short" }).format(dateLike);