Time Conversion Guide

Time is the dimension along which events are ordered. The SI unit is the second, defined since 1967 by the resonant frequency of the caesium-133 atom: one second is exactly 9,192,631,770 oscillations of radiation between two hyperfine levels of the caesium ground state. Every other working time unit is a fixed multiple of seconds: a minute is 60 s, an hour is 3,600 s, a day is 86,400 s. None of those is approximate.

Longer units are different. A week is exactly 7 days (a cultural artifact, not a physical period), so 1 week = 604,800 s exactly. But a month is not a fixed number of days — it ranges from 28 (February) to 31 (January, March, etc.), with a Julian-calendar average of 30.4375 days. Our converter handles the long-term scale by introducing the Julian year (365.25 days) and Julian month (year ÷ 12), so that 'years' and 'months' map to exact numbers of seconds.

The second has migrated from astronomy to atomic physics over a hundred and fifty years. Until 1956 it was 1/86,400 of a mean solar day. From 1956 to 1967 it was 1/31,556,925.9747 of the tropical year 1900 — a tidier definition, but still tied to Earth's orbit. In 1967 the General Conference on Weights and Measures redefined the second in terms of caesium-133 hyperfine transitions, and the second has been an atomic unit ever since.

Coordinated Universal Time (UTC) tracks both atomic time and Earth's actual rotation. Because the Earth's spin is slowly slowing down, atomic seconds drift ahead of solar seconds by tens of milliseconds per year. Since 1972, UTC has occasionally inserted a leap second to keep solar and atomic clocks within 0.9 seconds of each other — 27 leap seconds have been added through 2025, all positive. In November 2022 the General Conference on Weights and Measures resolved to retire the leap-second insertion by 2035, letting UTC drift further from solar time and removing a known source of software bugs. The exact replacement scheme is still under negotiation as of 2026.

Our time category supports twelve units. Sub-second precision: nanosecond, microsecond, millisecond. Working units: second (SI base), minute, hour, day, week. Long-term averages: Julian month (≈ 30.4375 days), Julian year (365.25 days), decade (10 Julian years), century (100 Julian years). The Julian month and Julian year are deliberate — they fix the average length of a month and year so that 'how many seconds in 5 years' has a single answer, even though no specific 5-year span has exactly 5 × 31,557,600 s in it.

If your question is calendar-aware (how many days between two specific dates, how old someone is in years-months-days, when a 30-year loan matures), this converter is the wrong tool. Use the date-difference calculator or the age calculator — both walk the real calendar, including leap years and month length.

Every time conversion routes through the second. Each unit declares how many seconds it represents: 1 minute = 60, 1 hour = 3,600, 1 day = 86,400, 1 week = 604,800, 1 Julian month = 2,629,800, 1 Julian year = 31,557,600. The methodology page lists each factor.

Useful exact factors: 1 day = 86,400 s. 1 week = 7 days = 604,800 s. 1 Julian year = 365.25 days = 31,557,600 s. These are the constants you actually need for back-of-envelope work. For 'how many seconds in a normal calendar year', the answer is 365 × 86,400 = 31,536,000 — about 21,600 seconds less than a Julian year, because the Julian year averages in the extra leap-day quarter.

Programming, billing, and project planning drive most time searches. Seconds to minutes handles log file analysis and runtime reports. Minutes to hours is the most common time conversion in everyday use — meeting durations, video lengths, cooking times. Hours to days is for shift planning and travel itineraries. Days to weeks handles sprint cycles and short project timelines. For long-horizon estimates (project budgets quoted in months, retirement planning quoted in years), the Julian-month and Julian-year units give average-length answers — and the compound-interest calculator wraps that math into a finance-aware tool.

Programmers reach for milliseconds and nanoseconds. A 60 Hz display refreshes every 16.667 ms; a 144 Hz display every 6.944 ms; a 1 GHz CPU clock ticks every 1 ns. Audio sampling at 44.1 kHz puts one sample every 22.676 μs. These sub-second numbers are the units of latency budgets, frame timing, and real-time signal processing.

The full time comparison table lists every supported unit with its second factor and a worked sample. Use it when you are pricing storage retention (seconds versus years), planning a project (hours versus weeks), or sanity-checking a benchmark that quotes microseconds alongside frames per second.

Months are not 30 days: a month length varies from 28 to 31 days. If you compute '6 months' as 180 days, you will be wrong by 1–3 days depending on which six months. Calendar-aware date math uses the date-difference calculator, which walks each month at its real length.

Years are not 365 days on average: the Gregorian year averages 365.2425 days; the Julian year used by our converter averages 365.25 days. The two differ by 11 minutes per year, which compounds to about three days every 400 years — the reason for the leap-year exception (years divisible by 100 but not by 400 are not leap years).

Leap seconds: UTC has had 27 positive leap seconds inserted since 1972, all without warning to legacy software. The General Conference on Weights and Measures decided in 2022 to retire leap-second insertion by 2035. If your software has special-cased leap seconds, the post-2035 behaviour is still being negotiated.

Business days vs calendar days: '5 days' in a contract often means 5 business days, not 5 calendar days, and the difference can be 7 days or more across a holiday weekend. We do not model business calendars — that is a contract-specific question, not a unit-conversion one.

Fortnights, scores, and lustra: the fortnight (14 days) and the lustrum (5 years) appear in older legal language and astronomy. Modern usage is essentially zero. We omit them rather than encourage confusion. If you need them, the conversion is simple: 1 fortnight = 1,209,600 s; 1 lustrum = 5 Julian years = 157,788,000 s.

Side-by-side: the full time comparison table lists every supported unit with its factor and a worked sample. Definitions: glossary entries for second, minute, and hour. Calendar-aware math: the date-difference calculator and age calculator compute real durations between specific dates. Method and sources: the methodology page cites the BIPM SI Brochure for the SI definition of the second; the 2022 CGPM resolution on retiring leap-second insertion by 2035 is a separate timekeeping decision that does not affect SI second arithmetic.