Convert Time Units

The astronomical second (1/86,400 of a mean solar day) was replaced in 1967 because Earth's rotation is irregular — it slows by about 2 milliseconds per century due to tidal friction. The cesium-133 atomic clock maintains accuracy to within one second over 300 million years, making it reliable enough for GPS navigation, telecommunications, and modern physics experiments.

The 365.25-day Julian year is a practical average that accounts for leap years. The actual tropical year (equinox to equinox) is approximately 365.2422 days. The Gregorian calendar corrects this by skipping leap years in most century years (1900, 2100) but keeping them in years divisible by 400 (2000, 2400). For most time conversions, the 365.25-day approximation introduces less than 0.01% error.

A mean month equals 30.4375 days (365.25 days ÷ 12), which is what time converters use. Calendar months vary from 28 to 31 days. This distinction matters in contracts, billing cycles, and project planning — a "3-month" deadline could mean 89 to 92 days depending on which months are involved, but the mean conversion gives 91.3 days.

A millisecond (0.001 seconds) is the typical latency of a fast internet connection and the refresh interval of computer displays. A microsecond (0.000001 seconds) is the timescale at which CPU instructions execute and GPS satellites synchronize. Even small timing errors compound: a 1-microsecond GPS clock error translates to approximately 300 meters of position error.