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.