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### B.1.1 Times

Functions for conversion of time values between various forms. The forms used are

Modified Julian Date (MJD)
  A continuous measure in days since midnight at the start of 17 November 1858. Based on UTC.
ISO 8601
  A string representation of the form `yyyy-mm-ddThh:mm:ss.s`, where the `T` is a literal character (a space character may be used instead). Based on UTC.
Julian Epoch
  A continuous measure based on a Julian year of exactly 365.25 days. For approximate purposes this resembles the fractional number of years AD represented by the date. Sometimes (but not here) represented by prefixing a 'J'; J2000.0 is defined as 2000 January 1.5 in the TT timescale.
Besselian Epoch
  A continuous measure based on a tropical year of about 365.2422 days. For approximate purposes this resembles the fractional number of years AD represented by the date. Sometimes (but not here) represented by prefixing a 'B'.
Decimal Year
  Fractional number of years AD represented by the date. 2000.0, or equivalently 1999.99recurring, is midnight at the start of the first of January 2000. Because of leap years, the size of a unit depends on what year it is in.

Therefore midday on the 25th of October 2004 is `2004-10-25T12:00:00` in ISO 8601 format, 53303.5 as an MJD value, 2004.81588 as a Julian Epoch and 2004.81726 as a Besselian Epoch.

Currently this implementation cannot be relied upon to better than a millisecond.

isoToMjd( isoDate )
  Converts an ISO8601 date string to Modified Julian Date. The basic format of the `isoDate` argument is `yyyy-mm-ddThh:mm:ss.s`, though some deviations from this form are permitted:
  - The '`T`' which separates date from time can be replaced by a space
  - The seconds, minutes and/or hours can be omitted
  - The decimal part of the seconds can be any length, and is optional
  - A '`Z`' (which indicates UTC) may be appended to the time
  Some legal examples are therefore: "`1994-12-21T14:18:23.2`", "`1968-01-14`", and "`2112-05-25 16:45Z`".
dateToMjd( year, month, day, hour, min, sec )
  Converts a calendar date and time to Modified Julian Date.
dateToMjd( year, month, day )
  Converts a calendar date to Modified Julian Date.
decYearToMjd( decYear )
  Converts a Decimal Year to a Modified Julian Date.
mjdToIso( mjd )
  Converts a Modified Julian Date value to an ISO 8601-format date-time string. The output format is `yyyy-mm-ddThh:mm:ss`.
mjdToDate( mjd )
  Converts a Modified Julian Date value to an ISO 8601-format date string. The output format is `yyyy-mm-dd`.
mjdToTime( mjd )
  Converts a Modified Julian Date value to an ISO 8601-format time-only string. The output format is `hh:mm:ss`.
mjdToDecYear( mjd )
  Converts a Modified Julian Date to Decimal Year.
formatMjd( mjd, format )
  Converts a Modified Julian Date value to a date using a customisable date format. The format is as defined by the `java.text.SimpleDateFormat` class. The default output corresponds to the string "`yyyy-MM-dd'T'HH:mm:ss`"
mjdToJulian( mjd )
  Converts a Modified Julian Date to Julian Epoch. For approximate purposes, the result of this routine consists of an integral part which gives the year AD and a fractional part which represents the distance through that year, so that for instance 2000.5 is approximately 1 July 2000.
julianToMjd( julianEpoch )
  Converts a Julian Epoch to Modified Julian Date. For approximate purposes, the argument of this routine consists of an integral part which gives the year AD and a fractional part which represents the distance through that year, so that for instance 2000.5 is approximately 1 July 2000.
mjdToBesselian( mjd )
  Converts Modified Julian Date to Besselian Epoch. For approximate purposes, the result of this routine consists of an integral part which gives the year AD and a fractional part which represents the distance through that year, so that for instance 1950.5 is approximately 1 July 1950.
besselianToMjd( besselianEpoch )
  Converts Besselian Epoch to Modified Julian Date. For approximate purposes, the argument of this routine consists of an integral part which gives the year AD and a fractional part which represents the distance through that year, so that for instance 1950.5 is approximately 1 July 1950.
unixMillisToMjd( unixMillis )
  Converts from milliseconds since the Unix epoch (1970-01-01T00:00:00) to a modified Julian date value
mjdToUnixMillis( mjd )
  Converts from modified Julian date to milliseconds since the Unix epoch (1970-01-01T00:00:00).

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TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note253
TOPCAT web page: http://www.starlink.ac.uk/topcat/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@bristol.ac.uk