Horology

The science of timekeeping is known as horology.
Nanosecond and Picosecond - A nanosecond is one billionth of a second, and a picosecond is one trillionth or 0.000 000 000 001 of a second.

Planck time - Planck time is the shortest known time span. It is the time it takes for light to travel a Planck length or 1.616199 × 10-35 meters in vacuum.

Easter celebration date - Easter is normally celebrated on the first Sunday after the first full moon that occurs on or after the Spring Equinox.

Light year - A light year is not a unit of time, but a unit of distance. The International Astronomical Union defines a light year as the distance light travels in vacuum in one Julian Year. In astronomy, a Julian Year corresponds to exactly 365.25 days.

Fortnight - A fortnight is a unit of time that refers to 14 days. It comes from an old English word, fēowertȳne niht, meaning fourteen night. It is commonly used in the UK, Ireland, and many commonwealth countries. People in the US and most parts of Canada use the term biweekly to refer to the time period of two weeks.

New York minute - The phrase in a New York minute refers to a very short period of time or an instant. Legend has it that the phrase originated in Texas in the late 1960s. The phrase was popularized by TV personality Johnny Carson who joked that a New York minute was the time between a traffic light turning green and the car behind one's car honking.

Jiffy - Jiffy is usually used to indicate a very short period of time, but it is formally defined in the fields of Physics and Chemistry as the time required for light to travel a centimeter. Also known as a light centimeter, a jiffy is equal to about 33.3564 picoseconds.

Friday 13th - Any month in the Gregorian Calendar that begins on a Sunday will have a Friday, the 13th, and there is at least one Friday the 13th in every year. A single calendar year can have up to 3 Friday the 13ths.