Convert between milliseconds, seconds, minutes, hours, days, weeks and years. Why there is no month unit and which year this uses.

Every time unit, one page

Pick any two units above. The conversions people look up most often have dedicated pages with reference tables, linked below.

The factors

Unit	Seconds
Millisecond	0.001
Second	1
Minute	60
Hour	3600
Day	86,400
Week	604,800
Year	31,557,600

Everything up to the week is exact and uncontroversial. The year is not, and there is no month at all.

Which year

This converter uses the Julian year of exactly 365.25 days — 31,557,600 seconds. It is the conventional constant for this purpose, and it is not any particular calendar year:

	Days	Seconds
Julian year	365.25	31,557,600
Gregorian mean year	365.2425	31,556,952
Tropical year	365.24219	31,556,925
Common year	365	31,536,000
Leap year	366	31,622,400

The gap between the Julian and Gregorian figures is 648 seconds — about eleven minutes a year, which is exactly the error that made the Gregorian reform necessary in 1582.

For durations of a few years the choice is immaterial. For anything astronomical, or for interest calculations over decades, it is not.

Why there is no month

A month is not a fixed length. It is 28, 29, 30 or 31 days depending on which one and which year, so “one month” cannot be converted into seconds without knowing which month.

Software that offers a month unit has quietly picked one of these:

Convention	Days	Used by
30 days	30	rough estimates, some billing
Average Gregorian month	30.436875	statistical work
Actual calendar month	28–31	anything that must be correct

Date arithmetic that has to land on the right day should add calendar months rather than seconds. “One month after 31 January” has no obvious answer, and every library resolves it differently.

Leap seconds

Since 1972, 27 leap seconds have been inserted into UTC to keep it in step with the Earth’s rotation, which is gradually slowing and does so unpredictably. A day containing one is 86,401 seconds long.

Unix time ignores them entirely — it is defined as if every day had exactly 86,400 seconds — which means Unix timestamps and true elapsed SI seconds have drifted apart by those 27 seconds. For almost every purpose this does not matter; for satellite navigation and high-frequency trading it does.

The international timekeeping bodies have agreed to stop inserting leap seconds by 2035.

Reference durations

	Seconds
Human blink	0.1–0.4
Typical web page load target	2.5
Football match	5400
Working day	28,800
Day	86,400
Week	604,800
30 days	2,592,000
Julian year	31,557,600

A useful coincidence worth knowing: a Julian year is very close to π × 10⁷ seconds. π × 10⁷ is 31,415,927, which is 0.45% short — good enough for order-of-magnitude physics and a standard trick for it.

Time converter