Time Unit Converter

About the Time Unit Converter

The time converter translates between ten units of time: millisecond, second, minute, hour, day, week, month (30 days), year (365 days), decade, and century. All conversions are expressed in seconds as the pivot unit, with the second being the SI base unit of time.

The second is defined by fixing the caesium-133 hyperfine transition frequency at exactly 9,192,631,770 Hz, per the 2019 SI redefinition. This means one second is the duration of 9,192,631,770 periods of the radiation emitted by a caesium atom at absolute zero. The definition, originally proposed in 1967, makes the second the most precisely realised of all SI base units.

The minute (60 seconds) and hour (3600 seconds) are non-SI units accepted for use with SI per the SI Brochure 9th edition, Table 8. They date back to the Babylonian sexagesimal system, the same system that gave us 360 degrees in a circle. The day (86400 seconds) is the mean solar day, though the actual length of the solar day varies slightly due to tidal friction (currently by about 1.7 milliseconds per century).

The month and year used here are the "30-day month" (2592000 seconds) and the "calendar year" (31536000 seconds, ignoring leap years). These are approximations: the actual solar year is about 365.2422 days, which is why leap years and leap seconds are needed. For precise astronomical time conversion, use a dedicated tool that accounts for leap years and leap seconds.

How It Works

The converter uses the two-step method through seconds:

base_in_seconds = value * factor[from]
result = base_in_seconds / factor[to]

The factor table (in seconds) is:

ms  = 0.001 s        s   = 1 s            min = 60 s
h   = 3,600 s         d   = 86,400 s         wk = 604,800 s
mo  = 2,592,000 s (30 days)    yr = 31,536,000 s (365 days)
dec = 315,360,000 s  cen = 3,153,600,000 s

The metric factors (ms, s) are exact. The minute, hour, and day are exact by definition (1 day = 24 hours = 1440 minutes = 86400 seconds). The week is exactly 7 days = 604,800 seconds.

The month and year are conventional approximations. The 30-day month (2,592,000 seconds) is used in many financial calculations. The 365-day year (31,536,000 seconds) ignores leap years; the actual average year length is 365.2425 days, or 31,556,952 seconds. The decade (10 years) and century (100 years) are derived from the 365-day year.

The two-step method through seconds keeps the code simple. The pivot unit also matches the SI convention, which expresses time intervals in seconds. For very long intervals (millennia or geological ages), use scientific notation in the result.

Worked Examples

Converting 1 year to seconds: the input in seconds is 31,536,000, displayed directly. This is the number of seconds in a 365-day year. The actual number of seconds in an average year (including leap years) is 31,556,952, which is about 0.066 percent larger.

Converting 1 day to minutes: the input in seconds is 86,400, and the result is 86,400 / 60 = 1440 minutes. This is the number of minutes in a 24-hour day, which is why a clock dial is divided into 12 hours of 60 minutes each.

Converting 1 century to years: the input in seconds is 3,153,600,000, and the result is 100 years. This is the number of years in a century, by definition. Converting to days gives 36,500 days (ignoring the 24 or 25 leap days that would actually occur).

When to Use This Tool

Use the time converter when you need to:

• Convert file timestamps from milliseconds to seconds or minutes.
• Translate billing intervals from minutes to hours for time tracking.
• Convert project durations from weeks to days for scheduling.
• Translate age or historical intervals from years to decades or centuries.
• Compute the number of seconds in a given number of days for timeout calculations.
• Convert astronomical time intervals from years to seconds for physics.
• Translate response-time targets from milliseconds to seconds for service-level agreements.

Limitations & Disclaimer

This converter uses fixed conversion factors and does not account for leap years, leap seconds, daylight saving time, or time-zone offsets. The "month" is defined as 30 days and the "year" as 365 days, which are approximations. For precise astronomical time conversion, use a tool that handles leap years and leap seconds. For clock-time conversion across time zones, use a dedicated time-zone converter. See our disclaimer for full details.

Frequently Asked Questions

Last updated: September 9, 2026  ·  Author: HT99 Tools Editorial Team