Time Unit Converter

Using the Time Unit Converter

1. Pick the source unit - millisecond, second, minute, hour, day, week, month (30-day approximation) or year (365-day approximation) - from the "From" dropdown.
2. Type the amount. The result recomputes on every keystroke through a Preact onInput handler.
3. Pick the target unit from the "To" dropdown.
4. Swap with the arrow button for the inverse.
5. Copy the answer to the clipboard through the browser Clipboard API.

What Is Actually Being Computed

Every unit is stored internally as a ratio to the second, the SI base unit of time (SI Brochure, BIPM 9th ed.). Since 1967 the second has been defined as exactly 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of caesium-133 - a definition pinned at CGPM 1967, reaffirmed in 2019 and in active use in every atomic clock on Earth and orbit. Minute = 60 s, hour = 3600 s, day = 86,400 s - all exact. Week = 7 days exact. Month and year in this tool use the flat approximations 30 days and 365 days respectively; the real Gregorian calendar month has a variable length between 28 and 31 days, and the mean tropical year is 365.24219 days. For calendar-aware arithmetic (birthdays, contract terms, lease expirations), this tool is the wrong instrument - use a date-difference calculator. For duration reasoning (how many seconds in a 5-hour video, how many milliseconds in a two-minute interval), the approximations are irrelevant.

When to Reach For It

• Turning a "video encoded in milliseconds" framerate into seconds for an editorial note.
• Converting server uptime in seconds (os.uptime()) into days and hours for an operations summary.
• Turning an HTTP max-age Cache-Control value in seconds into a human-readable week or month.
• Estimating an engineer-weeks number from hours of effort logged in a time-tracker.
• Reading a scientific instrument that times events in microseconds or nanoseconds and translating into the microsecond-to-millisecond scale.
• Converting between units in a physics homework when the textbook uses days but the formula expects seconds.

Pitfalls

Calendar reality breaks every long-range time conversion. A Gregorian year averages 365.2425 days because of the leap-year rule (365 * 400 + 100 - 4 + 1 = 146,097 days per 400-year cycle). A "year" in radiation dosimetry is exactly 365.25 days (the Julian year, about 31,557,600 s). A tropical year is 365.24219 days. A sidereal year is 365.25636 days. This converter picks 365 days for simplicity; if you need astronomical precision, use 365.25 days (Julian year) or ask a dedicated ephemeris. A month at 30 days is a common contract convention (30/360 in finance) but differs from any real Gregorian month. Finally, leap seconds: the SI second is steady, but UTC occasionally gets a leap second to track Earth rotation; the IERS Bulletin C announces these. The converter ignores leap seconds - it is arithmetic, not civil time.

Short History of the Second

For centuries the second was defined as 1/86,400 of a mean solar day - the rotation of the Earth. Improving pendulum clocks in the 17th century, then Shortt clocks in the 1920s, revealed that Earth rotation was irregular. In 1956 CGPM redefined the second as a fraction of the tropical year 1900 (ephemeris time). In 1967 the 13th CGPM adopted the caesium-133 definition, turning the second into the first SI unit tied to an atomic constant rather than an astronomical phenomenon. NIST-F2, the US primary frequency standard, uses a fountain of laser-cooled caesium atoms and reaches an uncertainty of about 1 part in 10^16. Future optical-lattice clocks based on strontium or ytterbium may push the definition to 10^-18; CGPM is expected to revisit the definition in the mid-2030s.

Alternatives

For human-readable durations, libraries like moment.js (legacy), date-fns or the Temporal API (now shipping in Firefox and Chromium) handle calendar-aware arithmetic. For scientific timing, Python\'s time.time_ns() returns nanosecond wall-clock time, and UNIX time/perf utilities measure program runtimes down to microseconds. GPS time, TAI (International Atomic Time) and UTC differ by integer seconds (37 s between TAI and UTC as of early 2024); a dedicated clock-comparison tool is better than this converter when you need to translate between timescales. This page is the right choice for unit-only scalar arithmetic.