Convert Days (d) to Years (yr)

Atomic time is based on the vibrations of cesium atoms and provides the most accurate timekeeping. Solar time is based on Earth's rotation relative to the Sun and varies slightly. Coordinated Universal Time (UTC) combines both, using atomic precision with occasional leap seconds to stay synchronized with Earth's rotation.

Leap years occur every 4 years (with exceptions for century years not divisible by 400) to account for Earth's 365.2425-day orbital period. This adds an extra day (February 29) every leap year. For precise yearly calculations, use 365.2425 days per year rather than 365 days.

Unix time counts seconds since January 1, 1970, 00:00:00 UTC (the 'epoch'). To convert Unix timestamp to human time, add the seconds to the epoch date. For example, Unix time 1609459200 equals January 1, 2021, 00:00:00 UTC. Most programming languages have built-in functions for this conversion.

Modern atomic clocks can measure time to within a few nanoseconds per day. Computer systems typically measure in nanoseconds or microseconds. For everyday use, millisecond precision is usually sufficient. GPS systems require nanosecond precision due to relativistic effects.

The current calendar system (Gregorian) evolved from Roman calendars. Originally, the year had 10 months starting in March. January and February were added later. Various political and practical decisions led to the irregular month lengths we use today, with months ranging from 28-31 days.

Time zones divide Earth into 24 regions, each roughly 15 degrees of longitude wide. When converting between times in different zones, you must account for the offset from UTC. For example, EST is UTC-5, so 12:00 UTC equals 7:00 AM EST. Daylight saving time adds additional complexity.

For 12-hour to 24-hour: Add 12 to PM hours except 12 PM (stays 12). For AM hours, keep the same except 12 AM becomes 00. For 24-hour to 12-hour: Hours 00-11 are AM (00 becomes 12), hours 12-23 are PM (subtract 12 except for 12).