📅 Date Reference Tools
Reference tools for developers and planners: ISO week numbers, day-of-year counts, leap year checker, Unix timestamp converter, and time zone converter. Precise calendar and timestamp information.
Calendar Systems and Date Standards
The Gregorian calendar, introduced by Pope Gregory XIII in 1582, is the international civil standard used by most of the world. It replaced the Julian calendar, which had accumulated an error of about 10 days due to a slightly inaccurate leap year rule. The Gregorian calendar's 365.2425-day average year closely matches the 365.2422-day solar year.
ISO 8601 is the international standard for representing dates and times. It specifies formats like YYYY-MM-DD (e.g., 2024-03-15), ordinal dates (2024-075 for the 75th day of 2024), and week dates (2024-W11-5 for the 5th day of week 11 in 2024). The ISO 8601 format is unambiguous — unlike "03/04/2024" which could mean March 4 or April 3 depending on your locale.
Unix Timestamps: The Developer's Date Format
Unix timestamps are the most common date format in software development. They represent a single point in time as an integer, making date arithmetic simple (add seconds for future dates, subtract for past dates), database storage efficient (a single 64-bit integer), and comparison trivial (larger integer = later time, regardless of time zone).
Key Unix timestamp reference points: January 1, 2000 = 946684800; January 1, 2020 = 1577836800; January 1, 2025 = 1735689600. JavaScript's Date.now() returns milliseconds, while most Unix systems and APIs use seconds. Always check which unit an API expects — passing milliseconds when seconds are expected (or vice versa) causes dates to be off by a factor of 1,000.
Time Zones: A Complex System
There are currently about 38 distinct UTC offsets in use worldwide, ranging from UTC-12:00 (Baker Island) to UTC+14:00 (Line Islands). Most time zones are whole-hour offsets, but India (UTC+5:30), Nepal (UTC+5:45), Iran (UTC+3:30), and several others use fractional offsets.
Daylight Saving Time (DST) adds complexity: roughly 70 countries observe DST, advancing clocks by 1 hour in summer. DST transitions are not uniform — different countries change on different dates, and some regions within a country observe different rules. The IANA Time Zone Database (also called tz database or tzdata) is the authoritative source for all time zone rules and historical changes, used by operating systems, programming languages, and our date tools.
Week Numbering Systems
There are multiple week numbering conventions: ISO 8601 (weeks start Monday, week 1 contains January 4) is the international standard used in Europe and international business. The North American system (weeks start Sunday, January 1 is always in week 1) differs significantly — the same date can have different week numbers in the two systems. Our week calculator uses ISO 8601 numbering by default but clearly labels the convention used.
Frequently Asked Questions
What is an ISO week number?
ISO week 1 is the week containing the first Thursday of the year. Weeks run Monday–Sunday. A year has 52 or 53 ISO weeks. January 1 can fall in week 52 or 53 of the previous year if it's a Friday, Saturday, or Sunday. Used in European business and international supply chains.
What is a Unix timestamp?
A Unix timestamp is seconds elapsed since January 1, 1970 00:00:00 UTC. Example: Jan 1, 2024 = timestamp 1704067200. JavaScript uses milliseconds (1704067200000). Unix timestamps are timezone-agnostic and used extensively in programming and databases.
What is the Year 2038 Problem?
Systems using 32-bit signed integers for Unix timestamps overflow on January 19, 2038 at 03:14:07 UTC (max value 2,147,483,647). Modern 64-bit systems are immune. Legacy 32-bit embedded systems and old software may be affected.
How do time zones work and what is UTC offset?
Time zones are offsets from UTC. UTC+5:30 is India Standard Time; UTC-5 is Eastern Standard Time. Some zones use 30 or 45-minute offsets. Daylight Saving Time adds 1 hour seasonally in ~70 countries. There are ~38 distinct UTC offsets globally.
What is the day of year number?
Day of year (ordinal date) numbers each day 1–365 (or 1–366 in leap years) from January 1. Used in meteorology, astronomy, and scientific logging. Part of the ISO 8601 ordinal format: 2024-007 = January 7, 2024.