How this calculator works
This converter handles the everyday time units — seconds, minutes, hours, days, weeks — along with months and years, which need a stated convention because calendar months and years do not all have the same length.
The Gregorian calendar's leap-year rule (a leap day every 4 years, except centuries not divisible by 400) produces an average year length of exactly 365.2425 days when averaged over a full 400-year cycle. This calculator defines 1 year as that 365.2425-day average, and 1 month as exactly one-twelfth of it (30.436875 days). These are standard conventions for unit conversion, but they are averages — any specific calendar month or year will differ. For a calendar-exact span between two real dates, use the date difference calculator instead.
How it converts
result = value × (seconds per from-unit) ÷ (seconds per to-unit)Exact anchors: 1 week = 7 days · 1 average year = 365.2425 days · 1 average month = 365.2425 ÷ 12 = 30.436875 days365.2425 comes from the Gregorian leap-year rule: 365 + 1/4 − 1/100 + 1/400 = 365.2425.
Worked example: 90 days to weeks
- 90 days × 86,400 seconds/day = 7,776,000 seconds.
- 1 week = 7 × 86,400 = 604,800 seconds.
- 7,776,000 ÷ 604,800 ≈ 12.857 weeks.
Frequently asked questions
Why isn't a month exactly 30 days?
Calendar months range from 28 days (29 in a leap year) to 31 days, so there is no single correct length. This converter uses the average Gregorian month, 30.436875 days (one-twelfth of the average year), which is the standard convention when a fixed conversion factor is needed.
Why 365.2425 days in a year instead of 365.25?
365.25 is the average under the simpler Julian calendar (a leap year every 4 years, no exceptions). The Gregorian calendar refines this by skipping the leap day in century years not divisible by 400 (2000 was a leap year, but 1900 and 2100 are not), giving a more accurate average of 365.2425 days.
How many seconds are in a year?
Using the average Gregorian year: 365.2425 days × 86,400 seconds/day = 31,556,952 seconds. This is the figure commonly used in fields like astronomy and finance where a fixed 'year' constant is needed.
Should I use this for exact ages or deadlines?
No — for a person's age or the exact number of days between two real calendar dates, use a date-based calculator, since real calendars have specific leap years and month lengths. This converter is for converting a duration expressed as a number, not for measuring between two dates.
How do I convert weeks to days?
Multiply the number of weeks by 7, since 1 week is defined as exactly 7 days — the one time-unit relationship on this page that is a fixed constant rather than a calendar average. Reversing the worked example confirms it: 12.857 weeks × 7 ≈ 90 days, the same 90 days it started from.