Time Conversions

The second was not always defined by atoms. Until 1967, it was 1/86,400 of a mean solar day. The problem was that Earth's rotation is not constant. Tidal friction from the Moon slows it by about 2.3 milliseconds per century, and irregular core-mantle interactions cause unpredictable fluctuations. In 1967, the 13th General Conference on Weights and Measures redefined the second as 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the cesium-133 ground state. This atomic definition is accurate to within 1 second in 300 million years. Modern optical lattice clocks push that further, losing less than 1 second in 15 billion years.

Subdivisions of the second follow a strict decimal hierarchy: 1 millisecond is 10^-3 seconds, 1 microsecond is 10^-6, and 1 nanosecond is 10^-9. These matter in concrete ways. Network latency on financial trading systems is measured in microseconds; a 10-microsecond advantage in executing a stock trade can be worth millions over a year. GPS satellites must account for time differences as small as 38.6 microseconds per day caused by relativistic effects (clocks run faster in weaker gravity), or positioning errors would accumulate at roughly 11 km per day.

Larger time units are less tidy. Minutes and hours carry over from the Babylonian base-60 system, adopted around 2000 BCE. The 24-hour day was formalized by the Egyptians. Weeks have no astronomical basis; the 7-day cycle appears to come from the Babylonian practice of assigning one day to each of the seven classical celestial bodies (Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn). Months vary from 28 to 31 days, and a calendar year is 365 or 366 days. For scientific and astronomical calculations, the Julian year (exactly 365.25 days, or 31,557,600 seconds) provides a fixed standard. Astronomers use it to define the light-year: the distance light travels in one Julian year.

Calendar irregularities also create practical conversion problems. February has 28 days in common years and 29 in leap years. The Gregorian calendar corrects for the fact that Earth's orbital period is approximately 365.2422 days (not 365.25) by skipping leap years in century years not divisible by 400. The year 1900 was not a leap year, but 2000 was. For software developers, this means a "month" has no fixed number of seconds, and converting between months and days or seconds requires either specifying exact dates or using the Julian month convention (30.4375 days = 2,629,800 seconds).

Time zones and daylight saving time add further conversion complexity. A "day" is not always 24 hours: the day a country springs forward has 23 hours, and the fall-back day has 25. These adjustments are irrelevant for pure unit conversion but matter enormously for scheduling software, log correlation, and deadline calculations.

This converter handles 12 time units: nanosecond, microsecond, millisecond, second (SI base), minute, hour, day, week, Julian month (30.4375 days, one-twelfth of a Julian year), Julian year, decade, and century. All conversions are exact multiplications: 1 minute = 60 seconds, 1 hour = 3,600, 1 day = 86,400, 1 Julian year = 31,557,600 seconds. Reference: NIST SP 330, IAU recommendations on astronomical units.