Time Unit Converter

Some units are exact and some are not

Seconds, minutes, hours, days and weeks convert exactly: 60, 60, 24 and 7. Months and years do not. A month is 28 to 31 days and a year is 365 or 366, so any conversion involving them is an approximation unless anchored to specific dates. Tools that quote a month as exactly 30 days are making an assumption they rarely state, and it accumulates: twelve 30-day months is 360 days, five days short of a year.

The second is the only defined one

Since 1967 the second has been defined by caesium-133 atomic transitions, not by dividing a day. That matters because the Earth's rotation is not perfectly regular: it fluctuates and has been very gradually slowing. Every other time unit is defined in terms of the second, not the other way round.

Leap seconds and the mess they cause

Because Earth's rotation drifts against atomic time, leap seconds have occasionally been inserted to keep them aligned, producing a minute with 61 seconds. Systems handle this inconsistently, and leap seconds have caused real outages at large internet companies. Unix time deliberately ignores them by repeating a value rather than incrementing, which means Unix timestamps are not a true count of elapsed SI seconds. A decision has been taken to stop inserting leap seconds by 2035.

Not every day is 24 hours

Where daylight saving applies, one day each year has 23 hours and another has 25. Date arithmetic that adds 86,400 seconds to get tomorrow lands an hour out twice a year, which is a common and hard-to-reproduce bug. Adding one calendar day and adding 24 hours are different operations, and the difference only shows up on two dates.

Working time is its own unit

Business days, working hours and man-days are conventions rather than measurements. A working month is commonly 21 or 22 days, a working year 240 to 260 depending on the jurisdiction and holidays. Any conversion involving them needs the convention stated, because the figures differ by several percent between organisations.