فهي تنتسب إلى الساعة 12 (منصف النهار) في يوم 0 من يناير 1900, وهذا ينتسب إلى 31 ديسمبر 1899 .
<ref>See [http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/FORTRAN/spicelib/j1900.html NASA Jet Propulsion Laboratory 'spice' toolkit documentation, function J1900].</ref>
It is also usual now to specify on what time scale the time of day is expressed in that epoch-designation, e.g. often [[Terrestrial Time]].
In addition, an epoch optionally prefixed by "J" and designated as a year with decimals (2000 +x), where x is positive or negative and quoted to 1 or 2 decimal places, has come to mean a date that is an interval of x Julian years of 365.25 days away from the epoch J2000 = JD 2451545.0 (TT), still corresponding (in spite of the use of the prefix "J" or word "Julian") to the [[Gregorian calendar]] date of 2000 Jan 1 at 12h TT (about 64 seconds before noon UTC on the same calendar day).<ref name=sofa /> (See also [[Julian year (astronomy)]].) Like the Besselian epoch, an arbitrary Julian epoch is therefore related to the Julian date by