Date Formatter — Format Dates in Any Pattern

ISO 8601 is the international standard for date and time representation, published by the International Organization for Standardization. The key principle is that date components are ordered from largest to smallest: year, month, day — written as YYYY-MM-DD (e.g., 2026-04-08). Combined date-time in ISO 8601 uses a "T" separator: 2026-04-08T14:30:00. The timezone is indicated by a "Z" suffix for UTC (2026-04-08T14:30:00Z) or an offset (+09:00 for JST). ISO 8601 is used in databases, APIs, JSON payloads, HTML date inputs, and most modern software systems. Its strict ordering makes it unambiguous and machine-sortable: alphabetical and chronological order are identical, which makes ISO 8601 strings ideal for filenames, log entries, and database keys.

RFC 2822 is the date format used in email headers (Date: field) and HTTP headers (Last-Modified, Date, Expires). It follows the format: "Day, DD Mon YYYY HH:MM:SS ±HHMM" — for example, "Tue, 08 Apr 2026 14:30:00 +0900". The month is abbreviated in English (Jan, Feb, Mar, etc.) and the day of week is included. RFC 2822 is human-readable but not easily machine-sortable. It is relevant for developers working with email systems (SMTP, IMAP), HTTP caching headers, RSS/Atom feeds, and HTTP/1.1 response headers. The JavaScript Date constructor accepts RFC 2822 strings, making it useful when parsing timestamps from email or HTTP responses in web applications.

A Unix timestamp is the number of seconds (or milliseconds) elapsed since the Unix epoch — 00:00:00 UTC on January 1, 1970. Unix timestamps are timezone-independent integers, making them ideal for storing dates in databases, computing durations, and passing timestamps between systems in different time zones. For example, 2026-04-08T00:00:00Z = Unix timestamp 1744070400. JavaScript uses millisecond precision (Date.now() returns milliseconds), while most Unix systems use second precision. The maximum value for a 32-bit signed integer Unix timestamp is January 19, 2038 — the "Year 2038 problem" — which is why modern systems use 64-bit integers. This tool outputs both second and millisecond Unix timestamps simultaneously.

Japan uses two date notation systems: the Gregorian calendar (西暦) and the Japanese imperial era system (元号). In the Gregorian system, Japanese formal writing places the year first: 2026年4月8日 (2026-nen 4-gatsu 8-nichi). The era system uses era names tied to imperial reigns — Reiwa (令和) began May 1, 2019, so 2026 is Reiwa 8 (令和8年). Official Japanese government documents, contracts, and some financial institutions still use era notation. This tool outputs the Gregorian Japanese format (e.g., "2026年04月08日" for the long form and "2026/04/08" for the short form). When writing dates in Japanese business correspondence, use the full 年月日 format with the year first.

Date format conventions differ significantly between the US and most other countries. The US uses MM/DD/YYYY (e.g., 04/08/2026 for April 8), while most of Europe and Japan use DD/MM/YYYY or YYYY/MM/DD. This difference causes significant confusion: "04/08/2026" is April 8 in the US but August 4 in the UK. The slash format (YYYY/MM/DD) used in this tool for the "slash format" output is unambiguous because it places the year first — the same order as ISO 8601 but with slashes instead of hyphens. When sharing dates internationally in non-technical contexts, always spell out the month name or abbreviation (April 8, 2026 or 8 Apr 2026) to avoid any possibility of confusion between month-day and day-month interpretations.