Time Converter

About time units

Time is one of the seven SI base quantities, and the second is its base unit. For everyday use, seconds, minutes, hours, days, and years are all familiar, but they mix two completely different counting systems: metric-style decimal prefixes (milliseconds, microseconds, nanoseconds) for very short intervals, and the ancient Babylonian base-60 system (60 seconds to a minute, 60 minutes to an hour) for human-scale durations. Above the day, things become irregular: months run 28 to 31 days, years are 365 or 366 days, and a 'month' has no fixed length in seconds. The converter uses 2,629,746 seconds (≈ 30.44 days) as the average month, and 31,556,952 seconds (365.2425 days) as the average Gregorian year.

History & origin

The second was historically defined as 1/86,400 of a mean solar day. In 1967 it was redefined atomically: one second is exactly 9,192,631,770 oscillations of the cesium-133 atom's hyperfine ground-state transition — a definition so precise that atomic clocks drift by less than a second in 300 million years. The 60-based minute and hour descend from Babylonian astronomy dating back over 3,000 years. The seven-day week comes from ancient Near Eastern calendars, most likely linked to the four lunar phases of roughly seven days each.

Tips

The most common time-conversion mistake is treating a month as exactly 30 days. That assumption is off by up to a day per month, accumulating to several days over a year. For billing cycles, interest calculations, or anything where precision matters, use actual calendar dates rather than multiplying by 30 or 2,629,746. Similarly, a year is not exactly 365 days — the Gregorian average is 365.2425 days, which is why leap years exist.

Common questions

How is the second defined today?

Since 1967 the second has been defined by atomic physics: it is the duration of exactly 9,192,631,770 cycles of the radiation corresponding to the hyperfine transition of the cesium-133 atom in its ground state. This makes the second independent of astronomical observation and reproducible in any adequately equipped laboratory.

How many seconds are in a year?

A common Gregorian calendar year has 365 days = 31,536,000 seconds. A leap year has 366 days = 31,622,400 seconds. The average over the Gregorian 400-year cycle is 365.2425 days = 31,556,952 seconds, which is the value this converter uses.