Time Converters 14
Time should be the easiest unit to convert — 60 seconds per minute, 60 minutes per hour, 24 hours per day. Simple ratios, no messy decimals. Then someone asks "how many days in a month" and it all falls apart. A month isn't a real unit of time — it's 28, 29, 30, or 31 days depending on the month, the year, and whether Julius Caesar is involved. These 14 converters handle the exact conversions. The fuzzy ones come with warnings.
Read the Time Conversion Guide
Why a day isn't 86,400 seconds — Earth's rotation is slowing. The 27 leap seconds since 1972, GPS time's 18-second drift, the UNIX 2038 problem, and the Mars lander that confused seconds with milliseconds.
Open the full guide →Time: The One Unit System Everyone Agrees On — Until They Don't
Time conversion is deceptively simple. 60 seconds per minute, 60 minutes per hour, 24 hours per day — these ratios are universal, the only measurement system shared by every country and every culture on Earth. They trace back to the Babylonians, who used a base-60 (sexagesimal) counting system around 2000 BCE, and to the ancient Egyptians, who divided daylight into 12 hours based on sundial markings.
But time gets complicated when you zoom out. A month is not a fixed unit — it varies from 28 to 31 days. A year is 365 days most of the time, 366 in leap years — except century years not divisible by 400. A week is 7 days because of the Babylonian planetary week, which was later adopted by Jewish, Christian, and Islamic calendars. And leap seconds — added 27 times since 1972 to keep UTC aligned with Earth's slowing rotation — nearly broke the internet in 2012 when Reddit, LinkedIn, and Qantas all crashed on the same leap-second night. The leap second was officially abolished in 2035 by the General Conference on Weights and Measures.
Time Conversion Reference
1 minute = 60 seconds | 1 hour = 3,600 seconds
1 day = 86,400 seconds (except on leap-second days = 86,401 s)
1 week = 604,800 seconds | 1 month = 2,419,200 to 2,678,400 seconds
1 millisecond = 0.001 s — the unit of high-frequency trading and CPU clock cycles
For the complete story — leap seconds, calendar math, and the ISO 8601 standard — see the Time Conversion Guide.
Seconds, Minutes & Hours
Days, Hours & Weeks
Milliseconds & Months
Exact conversion factors
Time conversions look trivial until a project schedule or a lease runs into the difference between a calendar month and a mean month. The table separates defined intervals from averages.
| From | To | Exact factor | Basis |
|---|---|---|---|
| 1 minute | seconds | 60 | Exact |
| 1 hour | seconds | 3,600 | Exact, 60 minutes |
| 1 day | seconds | 86,400 | Exact, the mean solar day |
| 1 week | days | 7 | Exact |
| 1 Julian year | days | 365.25 | Exact, the basis of the light-year |
| 1 Gregorian year | days | 365.2425 | Exact, the calendar average |
| 1 mean month | days | 30.4375 | A Julian year divided by 12, used for amortization |
| 1 Julian year | seconds | 31,557,600 | Exact, 365.25 times 86,400 |
| 1 sidereal day | seconds | 86,164.0905 | Mean sidereal day, about 3 minutes 56 seconds short of a solar day |
| 1 hour | minutes | 60 | Exact |
Frequently asked questions
How many seconds are in a day?
86,400 exactly, which is 24 times 60 times 60. That is the mean solar day. An individual day can be a millisecond longer or shorter, which is why leap seconds are occasionally needed.
How many days are in a month on average?
30.4375 if you divide a Julian year of 365.25 days by 12, which is the convention behind most amortization schedules. A Gregorian calendar month averages 30.436875 days.
Why is a sidereal day shorter than a solar day?
By about 3 minutes 56 seconds. Earth rotates once relative to the stars in 23 hours 56 minutes 4 seconds, but it has also advanced along its orbit, so it must turn a little more before the Sun returns to the meridian.
How many weeks are in a year?
52 weeks and one day, or two days in a leap year. That is why a fixed date drifts by one weekday each year, and why the ISO 8601 standard allows some years to have 53 numbered weeks.