Time Converter — Convert Hours, Minutes, Seconds, Days

How Time Conversion Works

Time is the one dimension that moves in only one direction, and converting between time units is fundamental to scheduling, science, engineering, and computing. The base SI unit of time is the second, defined since 1967 by the vibrations of cesium-133 atoms: exactly 9,192,631,770 oscillations per second. This definition is stable to within one second per 300 million years, making atomic clocks far more accurate than any astronomical observation.

A minute is 60 seconds, an hour is 60 minutes (3,600 seconds), a day is 24 hours (86,400 seconds). These groupings trace back to ancient Babylonian mathematics, which used a base-60 (sexagesimal) number system. The Babylonians chose 60 partly because it is divisible by 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, and 30 — more divisors than any smaller number — making fractions and subdivisions convenient without decimal notation.

Longer time units become less precise. A calendar year is 365 days, but Earth's actual orbital period (the tropical year) is approximately 365.2425 days. The Gregorian calendar accounts for this with leap years every four years, except century years not divisible by 400. For scientific calculations, a Julian year is standardized as exactly 365.25 days, which slightly overestimates the tropical year.

Time conversions are critical in computing (milliseconds and nanoseconds for processor cycles and network latency), physics (femtoseconds for laser pulses, light-travel-time for cosmic distances), finance (daily compound interest requires converting annual rates to per-second rates), and any global coordination across time zones where the same moment must be expressed in different local times.

Key Facts About Time

Sunlight travel time: Light takes approximately 8 minutes 20 seconds to travel from the Sun to Earth (about 149.6 million km). A signal to a Mars rover can take 3 to 22 minutes depending on the planets' orbital positions.
Leap seconds: Earth's rotation is gradually slowing due to tidal friction from the Moon. Leap seconds are occasionally added to Coordinated Universal Time (UTC) to keep atomic clock time aligned with astronomical time — 27 have been added since 1972.
CPU speeds: A 3 GHz processor completes one clock cycle in about 0.33 nanoseconds — one-third of a billionth of a second. Light travels about 10 centimeters (4 inches) in that time, which limits how physically large a processor can be.
Age of the universe: The universe is approximately 13.8 billion years old, or roughly 4.35 × 10¹⁷ seconds — a number that illustrates why scientific notation exists.

Frequently Asked Questions

How many seconds are in an hour?

There are exactly 3,600 seconds in one hour. This is because there are 60 minutes in an hour and 60 seconds in a minute: 60 × 60 = 3,600.

How many days are in a year?

A common year has 365 days. A leap year has 366 days (occurring every 4 years, with some exceptions). For conversions, an average year is approximately 365.2425 days or 31,556,952 seconds.

How many weeks are in a month?

On average, there are approximately 4.348 weeks in a month (52 weeks ÷ 12 months). Since months have different lengths (28–31 days), the exact number varies.

How do I convert hours to minutes?

To convert hours to minutes, multiply by 60. For example, 2.5 hours = 2.5 × 60 = 150 minutes. To convert minutes to hours, divide by 60.

How many milliseconds are in a second?

There are exactly 1,000 milliseconds in one second. A millisecond is one-thousandth of a second, often used in computing, sports timing, and audio engineering.