Time Unit Converter
Online time unit conversion tool - convert between years, months, weeks, days, hours, minutes, seconds, and milliseconds
What is Time Unit Conversion?
The Time Converter converts between units such as milliseconds, seconds, minutes, hours, days, weeks, months, and years. Common uses include runtime measurements, animation timing, API timeouts, cron planning, project estimates, contract periods, study schedules, and technical documentation. Some units are exact, while others depend on context: a day may not behave like exactly 24 hours around time zones and daylight saving changes, and months or years do not all have the same length. This tool is best for straightforward unit conversion, but calendar dates, billing periods, legal deadlines, and scheduling across time zones should still use date-aware calculations.
How to Use
How to use
- Enter the value to convert in the input field
- Select the source unit (From) and target unit (To)
- Click the Convert button or view the real-time result
- Click Related Conversions to see other unit combinations
Unit Assumptions
- Month and year conversions use average lengths unless the tool states otherwise; they are not the same as calendar date arithmetic.
- For billing, SLAs, or logs, keep milliseconds or seconds as the source value and record the rounding rule.
Use Cases
Technical Principle
The second is the SI base unit for time, redefined at the 13th General Conference on Weights and Measures in 1967 as 9,192,631,770 cycles of the radiation emitted by the hyperfine transition of the ground state of caesium-133 at 0 K. All higher units in this tool are integer multiples of the second: 1 minute = 60 s, 1 hour = 3,600 s, 1 day = 86,400 s, 1 week = 604,800 s. These factors come from the historical Babylonian sexagesimal system (base 60) and the Egyptian 24-hour day, not from physics, but they have been frozen by the SI brochure and ISO 80000-3:2019 so the conversion is exact. Years and months are inherently fuzzy. The Gregorian calendar (Inter gravissimas, Pope Gregory XIII, 1582) defines a mean year of 365.2425 days via the rule "divisible by 4, except centuries not divisible by 400" (1600 and 2000 leap, 1700/1800/1900 common). The Julian year of 365.25 days is still used in astronomy and is the convention this converter applies for year→day conversions because it matches the most common spreadsheet and library default. The mean month is then 365.2425 / 12 ≈ 30.436875 days, often rounded to 30.44; this converter uses 30.44. Calendar arithmetic ("add one month to January 31") cannot be done by adding a fixed number of seconds, so any business-day, billing, or contract calculation must fall back to a date-aware library such as date-fns, Luxon, or Temporal. POSIX time (Unix epoch) counts non-leap seconds since 1970-01-01 00:00:00 UTC and explicitly ignores the 27 leap seconds inserted by the IERS between 1972 and 2017 to keep UTC within 0.9 s of UT1. As of 2024 TAI - UTC = 37 s, and the CGPM voted in 2022 to abandon the leap second by or before 2035. JavaScript's Date.now() returns POSIX milliseconds with no leap-second adjustment, and performance.now() returns a monotonic high-resolution timestamp with sub-millisecond precision (browsers clamp to 5 µs to 1 ms for Spectre mitigation). IEEE 754 double precision can represent ±2^53 ms exactly, which covers roughly ±285,427 years from the epoch, far beyond any practical conversion this tool will see.
- SI definition: 1 second = 9,192,631,770 periods of caesium-133 hyperfine transition radiation (CGPM 1967, reaffirmed in the 2019 SI brochure)
- Fixed integer factors: 1 min = 60 s, 1 h = 3,600 s, 1 d = 86,400 s, 1 wk = 604,800 s; these are exact by ISO 80000-3 definition, not measurements
- Gregorian mean year: 365.2425 days (4/100/400 leap rule); Julian year: 365.25 days; this tool uses 365.25 d/yr and 30.44 d/mo (365.25 / 12 with rounding) as documented in the FAQ
- POSIX time ignores leap seconds: 27 inserted by IERS between 1972 and 2017; TAI - UTC = 37 s as of 2024; CGPM resolved in 2022 to retire the leap second by ~2035
- Sub-second units: 1 s = 10^3 ms = 10^6 µs = 10^9 ns; Planck time ≈ 5.39 × 10^-44 s is the theoretical lower bound for physically meaningful intervals
- JavaScript precision: Date.now() returns int64 milliseconds since 1970-01-01 UTC; performance.now() is monotonic with 5 µs-1 ms clamping per W3C High Resolution Time level 3 (Spectre mitigation)
- Calendar arithmetic limits: month and year additions are not associative (Jan 31 + 1 month + 1 month != Mar 31 in February); use the Temporal proposal or date-fns for civil-date math
Examples
Days to hours, minutes, and seconds
1 day = 24 h = 1,440 min = 86,400 s = 86,400,000 msHours to minutes and seconds
1 h = 60 min = 3,600 s; an 8-hour workday = 480 min = 28,800 sYears to days, hours, and minutes (average)
1 year ≈ 365.25 d ≈ 8,766 h ≈ 525,960 min ≈ 31,557,600 sWeek and month references
1 week = 7 d = 168 h = 10,080 min; 1 average month ≈ 30.44 d ≈ 730.5 hCommon year vs. leap year
Common year = 365 d = 8,760 h; leap year = 366 d = 8,784 h (one extra day every 4 years, minus century rules)Milliseconds in programming and animation
1 s = 1,000 ms; a 60 fps frame budget = 16.67 ms; a 1.5 s debounce window = 1,500 msFAQ
Which time units are supported?
Nanoseconds, microseconds, milliseconds, seconds, minutes, hours, days, weeks, months, years, decades, centuries, millennia. Pick any two units and the page converts in real time.
How is a 'month' or a 'year' defined?
Conversion uses average lengths: month = 30.44 days (365.25/12), year = 365.25 days (Julian year). Real months and years vary in length, so converting to or from these units is necessarily approximate. For exact calendar arithmetic use the Date Difference tool.
Why is a year 365.25 days, not 365?
Because the solar year is 365.2422 days. The Julian approximation rounds to 365.25 (one leap day every 4 years). The Gregorian calendar's 400-year average is 365.2425. For most conversions this 0.06% difference doesn't matter; for archaeological dates it accumulates.
Can I convert hours-minutes-seconds?
The page typically converts a single unit value (e.g. 5400 s ↔ 1.5 h ↔ 90 min). For HH:MM:SS-style display use a duration formatter; this tool gives you the underlying numeric conversion.
How precise are sub-second conversions?
1 second = 10^9 nanoseconds. JavaScript's number precision is 15-17 significant digits, so the converter handles roughly ±10^7 seconds of nanosecond precision before rounding kicks in. For atomic-clock work, use a specialist tool.
Is a 'leap second' considered?
No. The page treats every minute as exactly 60 seconds. Leap seconds (occasionally inserted to keep UTC aligned with Earth's rotation) are too rare to factor into general conversions.
Is the calculation done locally?
Yes. The unit conversion is pure JavaScript arithmetic in the browser; nothing is logged or sent over the network.