Time Converter

Time intervals are conventionally measured in seconds, minutes, hours, days, weeks, months, and years. The first four are exact; weeks are exactly 7 days; but months and years vary in length, which makes long-interval conversion approximate unless you specify which months and years.

The converter uses standard averages: 30.44 days per month (year ÷ 12), 365.25 days per year (Julian average). For exact calendar arithmetic across specific dates, use a date difference calculator instead.

The second is the SI base unit, defined since 1967 as 9,192,631,770 periods of the radiation from a cesium-133 transition. Minutes (60 s), hours (3,600 s), and days (86,400 s) are exact integer multiples by convention. The week (7 days) is also exact. From there it gets fuzzy: months range 28 to 31 days, years are 365 or 366. The Julian year (365.25 days) averages over the 4-year leap cycle. The Gregorian year (365.2425 days) accounts for the 100/400 century rule. Astronomers prefer the tropical year (365.24219 days), which tracks the actual solar cycle. The differences between these are smaller than a day per century but real.

A worked example. Convert 2.5 years to seconds. Using the Julian average: 2.5 × 365.25 × 86,400 = 78,840,000 s. Using exact calendar arithmetic from January 1, 2024 to July 1, 2026 (a span that includes one leap day): 78,883,200 s. The difference is about 12 hours — small in percentage terms but not zero. For billing, contracts, and calendar-aware applications, use exact dates. For physics and engineering where the absolute reference doesn't matter, the average is fine.

Limitations to keep in mind. Leap seconds are inserted into UTC occasionally to keep clocks aligned with Earth's slowing rotation. There have been 27 since 1972. Most calculators ignore them, which is correct for almost every application but matters for GPS timing, financial trading timestamps, and astronomical observation. The IERS announced in 2022 that leap seconds will be deprecated by 2035 in favor of letting UTC drift slightly from astronomical time. For very long intervals (centuries), the Earth's rotation is slowing, day length is increasing by about 1.7 ms per century, and historic dates require care to match observed astronomical events.