https://github.com/Xotic750/astrodate wikidata: https://meta.wikimedia.org/wiki/Wikidata/Data_model#Dates_and_times tzdb https://upload.wikimedia.org/wikipedia/commons/9/9f/Tz_map_world2009r_efeledotnet.png https://github.com/eggert/tz http://qntm.org/calendar http://www.cacr.caltech.edu/futureofutc/aas223/
solar time local apparent / true / sundial time based on apparent motion of sun crude
local mean / clock time early 1800s corrected by equation of time still solar still very local
standard time weather forecasting, steam engines, trains, telegraph synchronizes larger regions 1883 general time convention (US railroads) 1884 international meridian conference
GMT mean solar time on the prime meridian at royal observatory at greenwich london UK
UT / universal time based on rotation of earth in relation to stars / quasars at first, used ambiguously with GMT 1935 proposed as more precise term than GMT modern continuation /successor of GMT variable, years & days getting longer because earth is slowing because tides 1961-01-01 first form of UTC occasional steps of non-integer seconds seconds slightly longer than SI second second length periodically changed to approximate earth's rotation discontinuous composed from segments: linear transformations of atomic time discontinuities arranged smartly approximated UT2 (until 1972) leap seconds are generated by UT (astronomical) UT0 weird UT1 principal form determined by: quasar pulses ICRF / international celestial reference frame barycenter of the solar system laser ranging of moon GPS satellite orbits same everywhere on earth proportional to rotational angle of earth wrt quasars UT2 linear transformation of the BIH's atomic time time scale was stable and internationally synchronised 1972-01-01 UTC present form year usually has 86,400 SI seconds leap seconds keep it within 0.9 seconds of UT1 began tracking (approximating) UT1 instead of UT2 compromise arrangement for a broadcast time scale navigation requires a source of Universal Time navigation well served by public time broadcasts
sidereal time??? https://en.wikipedia.org/wiki/Greenwich_Sidereal_Time
ERA / Earth Rotation Angle determination of a measure of the Earth's angle with respect to the ICRF modern replacement for Greenwich Mean Sidereal Time allowed by observations (which?)
ET / ephemeris time attempt at dynamical time scale 1948 proposed to overcome the drawbacks of irregularly fluctuating mean solar time 1952 began 1955 cesium atomic clocks became operational 1956/1960 standard second 1958 ephemeris second defined as 9192631770±20 cesium resonance cycles 1958 cesium atomic clock calibrated by reference to ephemeris time 1967/8 SI second redefined as 9192631770 periods of ceasium 133 radiation 1970s superceded by TT / terrestrial time astronomical superceded ephemeris units of SI seconds (which is based on the ceasium atomic clock) but based on theoretical ideal, not any particular clock or atom indirectly underlies UTC via TAI differs from UT by delta-T, which is increasing parabolically 1976 IAU resolved is theoretical basis is non-relativistic 1984 astronomical almanac adopts high-precision ephemerides from NASA JPL T-eph relativistic coordinate time differs from Terrestrial Time only by small periodic terms with an amplitude not exceeding 2 milliseconds of time constant / linear offset of 0.5 sec/yr from TCB (1991) 1984 replaced by relativistic / dynamic timescales. "difficulties were realized" TDT / terrestrial dynamical time TDB / barycentric dynamical time 1991 IAU adopts BCT / TCB / barycentric coordinate time 1990s TT / terrestrial GCT / TCG / geocentric coordinate time
TAI 1955-07 Tm / AM begun by BIH / International Time Bureau caesium: local, compared to distant by VLF radio 1963 renamed A3 1969 renamed TA(BIH) 1956-09-13 A.1 / US Naval Observatory / Atomichrom 1958 BIH + A.1 defined by an epoch at the beginning of 1958 TAI synchronized with UT inevitably imperfect: UT2 observations off by several hundredths of a second Julian Date 2436204.5 (1 January 1958 00:00:00) at the corresponding UT2 instant 1971 based on SI seconds with no leap seconds, named TAI 1970s discovered gravitational time dilation, average of altitudes 1977-01-01 clocks corrected so that TAI == proper time at mean sea level (the geoid) TAI slowed down by ~10^-12 (clocks were mostly at high altitudes) EAL / Echelle Atomique Libre / Free Atomic Scale == older uncorrected scale epoch for TCB, TCG, TT based on the notional passage of proper time on Earth's geoid proper time is some relativistic thing that i don't understand basis of civic /broadcast / UTC: TAI corrected by leap seconds to avoid drift from solar basis of Terrestrial Time: used for astronomical calculations 200 atomic clocks in over 50 national laboratories worldwide compared using GPS signals and two-way satellite time and frequency transfer weighted average these were exactly the same second: 1972-01-01 00:00:00 UTC 1972-01-01 00:00:10 TAI
unix time seconds since 1970-01-01 00:00:00 UTC NOT counting leap seconds not a linear representation of time not a true representation of UTC
proper time? https://en.wikipedia.org/wiki/Dynamical_time_scale free-fall time? https://en.wikipedia.org/wiki/Second#International_second
possible frontend: http://wiki.openstreetmap.org/wiki/Deploying_your_own_Slippy_Map