About the Time Unit Converter

The time converter translates between milliseconds, seconds, minutes, hours, days, weeks, months, years, decades, and centuries. Whether you are timing a computer operation in milliseconds, planning a project in weeks, or comparing historical eras in centuries, this tool gives you an instant conversion in either direction.

The second is the SI base unit of time, defined as the duration of 9,192,631,770 periods of the radiation emitted by a caesium-133 atom. Every other unit in this converter is a defined multiple of the second. The minute, hour, and day are exact (60, 3600, and 86400 seconds respectively). The week is exact (7 days, 604800 seconds).

Months and years are not exact multiples of the second because they vary in length. A month can be 28, 29, 30, or 31 days, and a year can be 365 or 366 days. This converter uses the Julian year (exactly 365.25 days, or 31,557,600 seconds) and the Julian month (one-twelfth of a Julian year, about 30.44 days) as average values. These are the standard astronomical averages used in scientific calculations.

How It Works

Each unit has a defined relationship to the second, the SI base unit of time. The converter uses these factors to translate between any two units:

1 ms   = 0.001 s
1 s    = 1 s
1 min  = 60 s
1 h    = 3,600 s
1 d    = 86,400 s            (24 hours)
1 wk   = 604,800 s           (7 days)
1 mo   = 2,629,800 s         (Julian month, 1/12 of a Julian year)
1 yr   = 31,557,600 s        (Julian year, 365.25 days)
1 dec  = 315,576,000 s       (10 Julian years)
1 cen  = 3,155,760,000 s     (100 Julian years)

Every conversion proceeds in two steps: the input is multiplied by the source unit's factor to give a duration in seconds, then divided by the target unit's factor to give the result in the target unit. The factors for days and smaller are exact; the factors for months and larger use the Julian year as a standard average.

The Julian year of 365.25 days is the average length of a year in the Julian calendar, which alternates three 365-day years with one 366-day leap year. The Gregorian calendar most of the world uses today has a slightly different average year length (365.2425 days), but the Julian year remains the standard for astronomical calculations and is the basis for the light-year distance unit.

Worked Examples

To convert 1 day to seconds, the calculation is 1 * 86400 = 86400 seconds. To convert the same day to hours, the calculation is 86400 / 3600 = 24 hours. To convert the same day to milliseconds, the calculation is 86400 * 1000 = 86,400,000 milliseconds.

To convert 1 year to seconds, the calculation is 1 * 31557600 = 31,557,600 seconds (using the Julian year). To convert the same year to days, the calculation is 31557600 / 86400 = 365.25 days. This is the average year length used in astronomical calculations, which is why a light-year is the distance light travels in 31,557,600 seconds.

To convert 1 century to days, the calculation is 3155760000 / 86400 = 36525 days. This is 100 Julian years of 365.25 days each, which is the standard astronomical definition. A Gregorian century averages about 36524.25 days, slightly shorter because the Gregorian calendar omits three leap years every 400 years.

When to Use This Tool

Use the time converter whenever you need to translate between time units:

Converting computer execution times between milliseconds, seconds, and minutes.
Planning projects with durations measured in weeks, months, or years.
Converting scientific or astronomical time intervals.
Calculating age or service intervals in different units.
Working with media durations (audio, video) in different units.
Comparing historical eras measured in decades or centuries.
Teaching the relationships between time units and the calendar.

Limitations & Disclaimer

This converter uses the Julian year (365.25 days) and Julian month (1/12 of a Julian year) as standard averages. It does not convert calendar months (which vary from 28 to 31 days), calendar years (which can be 365 or 366 days), or specialised astronomical units such as the sidereal year or the galactic year. For date arithmetic, use a dedicated date calculator. See our disclaimer for full details.

Frequently Asked Questions

Why does the converter use 365.25 days for a year?

Because the Julian year of exactly 365.25 days is the standard astronomical average. It accounts for leap years by averaging three 365-day years with one 366-day year. The Gregorian calendar most of the world uses has an average year of 365.2425 days, slightly shorter, but the Julian year is the basis for scientific calculations and the light-year.

Why is a month defined as 1/12 of a Julian year?

Because calendar months vary in length from 28 to 31 days, there is no single exact value for a month in seconds. The Julian month (one-twelfth of a Julian year, about 30.44 days or 2,629,800 seconds) is the standard average used in scientific and engineering calculations. For specific calendar months, use a date calculator instead.

Is the second an exact unit?

Yes. The second is the SI base unit of time, defined as the duration of 9,192,631,770 periods of the radiation emitted by a caesium-133 atom at absolute zero. This definition, adopted in 1967, makes the second one of the most precisely defined units in the SI system, and all other time units are derived from it.

How many seconds are in a day?

Exactly 86,400 seconds, since one day is defined as 24 hours of 60 minutes of 60 seconds each. Leap seconds are occasionally added to keep clocks synchronised with the Earth's rotation, but they do not change the definition of a day in this converter.

What is the difference between a Julian and a Gregorian year?

The Julian year averages 365.25 days; the Gregorian year averages 365.2425 days. The Gregorian calendar omits three leap years every 400 years to better match the solar year (about 365.2422 days). For most practical purposes the difference is negligible, but for long-term astronomical calculations it accumulates.

How accurate are the conversions?

The conversions for seconds through weeks are exact. The conversions for months, years, decades, and centuries use the Julian year as a standard average, which is exact by definition. The only source of error is floating-point rounding, which is negligible for practical purposes.

Last updated: July 21, 2026 · Author: HT99 Tools Editorial Team · Reviewed by: HT99 Tools Editorial Team