How do we measure time?

Although rocks, trees, fishes, bugs, little furry creatures, and most of humanity reckon time by the rotation of the earth, physicists reserve the term for a more uniform concept. The irregularities in the rotation of the earth mean that no form of earth rotation time measures time in the sense used by physicists. All forms of earth rotation time are now better described as ``time-of-day. In the vocabularies of most of the languages of the world the long history of the words for ``time has been to mean ``time-of-day. In order for ``time to have a simple relationship with a calendar, that ``time must be a form of ``time-of-day. Steve Allen

name	basis	calculation	@ epoch	length	months	days	new year
International Atomic	-	rules
Gregorian	tropical solar	rules	1970-01-01
Assyrian	tropical solar	rules
Chinese Solar	solar	rules
Persian	solar	?	1348 Dey 11	.	12	.	vernal equinox
Chinese Lunar	solilunar	rules
Jewish	solilunar	.	5730 Teveth 23
Meyer-Palmen Solilunar	solilunar
Wikipedia:Rapa_Nui_calendar	lunisolar	.	.	.	13
Liberalia Triday	lunisolar
Goddess Lunar	solar-count lunar
Iranian
Tabular Islamic Calendar	lunar	rules
Muslim	lunar	observational	1389 Shawwal 22	12 month	lunar	variable
Babylonian
Balinese
Saka
Mayan	.	.	12-17-16-7-5
Indian Civil	.	.	1891 Pausa 11	.	12	vernal equinox

Nychthemeron Time

UT0 (UT-zero): Universal Time determined at an observatory by observation of stars, and also from ranging observations of the Moon and artificial Earth satellites. A measure of the earths ACTUAL rotation. Different observatories will find a different value for UT0 at the same moment. It is thus not, strictly speaking, Universal.
UT1: Computed from UT0 by applying a correction factor to account for the effect of polar motion on the longitude of the observing site. UT1R has tidal effects taken into account. UT1 is the same everywhere on Earth.

Siderial Time

Sidereal time is the measure of the earth's rotation with respect to distant celestial objects.

LST - Local Sidereal Time
LMST - Local Mean Sidereal Time
GAST - Greenwich Apparent Sidereal Time: GMST corrected for the shift in the position of the vernal equinox due to nutation.
GAST = GMST + (equation of the equinoxes)
GMST - Greenwich Mean Sidereal Time

Atomic Time

TAI - International_Atomic_Time: 86400 sec/day, ALWAYS.
UTC - Coordinated_Universal_Time: An atomic timescale that approximates UT1. It is the international standard on which civil time is based. As the rotation of the Earth is somewhat irregular, leap seconds are currently introduced when necessary to correct for any variance.
UTC = TAI - (number of leap seconds) Currently TAI is approximately 34 seconds ahead of UTC.
CT - Coordinate Time, or ET - Wikipedia:Ephemeris_time (1960-1984): 1 second = the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time.
TDB - Barycentric_Dynamical_Time (1984-1991): A timescale that increments uniformly in a reference frame co-moving with the barycenter of the solar system.
TDB = TT + 0.001658 sin( g ) + 0.000014 sin( 2g ) seconds
where g = 357.53 + 0.9856003 ( JD - 2451545.0 ) degrees and JD is the Julian Date.
TDT - Terrestrial Dynamic Time (1984-1991): TT = TAI + 32.184 = UTC + (number of leap seconds) + 32.184 on 1977-01-01T00:00:00 TAI
TCB - Barycentric_Coordinate_Time(1991-): based on a reference frame comoving with the barycentre of the Solar system, and is used for calculating motion of bodies within the Solar system. TCB differs from ET by only a constant offset and a constant rate. As such, TCB provides an equivalent alternative to ET, but it does not allow increased accuracy as others have implied. Because the reference frame for TCB is not influenced by the gravitational potential caused by the Solar system, TCB ticks faster than clocks on the surface of the Earth by about 1.6 × 10−8 (about 490 milliseconds per year). Consequently, the values of physical constants to be used with calculations using TCB differ from the traditional values of physical constants.
TCG - Geocentric_Coordinate_Time (1991-): Geocentric Coordinate Time is based on a reference frame comoving with the geocentre (the centre of Terra), and is used for calculations concerning planetary rotation and satellites. Because the reference frame for TCG is not rotating with the surface of the Earth and not in the gravitational potential of the Earth, TCG ticks faster than clocks on the surface of the Earth by about 7.0 × 10−10 (about 22 milliseconds per year). Consequently, the values of physical constants to be used with calculations using TCG differ from the traditional values of physical constants.

TT - Terrestrial_Time (1991-):New name for TDT. TT is a theoretical ideal, not dependent on a particular realisation. For practical purposes, TT must be realised by actual clocks in the Earth system.
(1991-): TT was defined to be a linear transformation of TCG, such that TT agrees with proper time on the geoid.
(2000-): TT = TCG x (1 - 6.969290134 × 10⁻¹⁰) + E. This has the effect of redefining the geoid in terms of a precise gravitational potential, thus removing the need for horologists to study sea levels.

MJD - Modified Julian Date: Defined in the 1950s as ( JD - 2400000.5 ). MJD 0.0 corresponds to 1858-11-17T00:00:00
Any current application which requires precision better than one minute, or any historical application which requires precision better than several hours, should take care to indicate which time scale is associated with the use of JD or MJD. It should also be noted that the use of JD or MJD for the UTC time scale is problematic and ambiguous at the precision of one second.
EAL: ?
ERA - Earth Rotation Angle or Theta: Beginning in 2003 UT1 is no longer determined from GMST, but from the Earth Rotation Angle (ERA).

'Other' Time

Unix Time: 86400 sec/day. Insertion of a leap-second achieved by decrementing counter. Result - Unix seconds are not 1.0 seconds long, but 1 day/86400.

The current time can be found at Multiple Time Display site.

Note on POSIX

"The main obstacle is POSIX. POSIX is a ``standard'' designed by a vendor consortium several years ago to eliminate progress and protect the installed base. The behavior of the broken localtime() libraries was documented and turned into a POSIX requirement.

Fortunately, the POSIX rules are so outrageously dumb---for example, they require that 2100 be a leap year, contradicting the Gregorian calendar---that no self-respecting engineer would obey them." DJB.

Software