Seconds through weeks convert exactly — a minute is always 60 seconds, a day is always 86,400. Months and years are a different story, and this calculator says so rather than hiding it.
How it works
Exact relationships
1 min = 60 s · 1 h = 3600 s · 1 day = 86,400 s · 1 week = 604,800 s
Why months and years are conventions, not exact units
A calendar month is 28, 29, 30 or 31 days depending on which one. A calendar year is 365 or 366 days depending on whether it is a leap year. Neither has one correct length in seconds — there is no such thing as “the” length of a month. Rather than silently pick 30 days for a month (defensible, and wrong for eight months of the year) or 365 for a year (defensible, and wrong every fourth year), this uses the Julian year — 365.25 days, 31,557,600 seconds — the astronomical convention astronomers themselves use specifically because it is a fixed, citable average rather than an arbitrary round number. A month here is exactly one twelfth of that. Both fields say plainly that they are an average, not a specific calendar month or year.
How to use this calculator
- Type a duration into any field.
- The rest update — days, hours and minutes up front; seconds, weeks, months and years behind “More units”.
- Treat the month and year fields as estimates for planning, not as a substitute for an actual calendar when the exact date matters — the date-difference calculator handles that case properly, counting real calendar days.
Frequently asked questions
Why isn’t a month just 30 days here?
Because 30 days is wrong for seven months of the year (the 31-day ones) and wrong in a different direction for February. There is no single correct number of days in “a month” — this uses a stated, citable average instead of an arbitrary round figure.
What is a Julian year, and why use it instead of 365 days?
365.25 days — the average length of a year once leap years are accounted for. Astronomers use it precisely because it is a fixed, agreed convention rather than a choice between “365, except when it isn’t.”
If I need an exact date, should I use this calculator?
Not for the month and year fields — those are averages. For an exact count of days between two real calendar dates, including correct handling of month lengths and leap years, use the time-between-dates calculator instead.
Are seconds, minutes, hours, days and weeks ever approximate?
No — those five are always exact multiples of each other, with no convention or averaging involved at any point.
