caldat

#!/bin/bash -e
[ -n "$DEBUG" ] && set -x

# ----------------------------------------------------------------------------
if [[ "$OSTYPE" == 'linux'* ]]; then
  TEMP=$(getopt -o f:hvV --long format:,help,version,verbose \
                --name "$0" -- "$@")
  [ $? != 0 ] && { echo "Terminating..." >&2; exit 1 ; }
  eval set -- "$TEMP"
fi

version(){
cat<<EOF
${0##*/} 2019.01.1

Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Written by Yaswant Pradhan.
EOF
}
usage(){
cat<<EOF
Get calendar date time from given julian day using Unix datetime.

Usage: ${0##*/} [OPTIONS] [JULDAY]

JULDAY  Input Julian day (see julday(1)). Default is for system date time.

Options:

  -f, --format STRING
      Specify format for output caledar date. Default is ’%FT%TZ’.
      See date(1) for valid date format specifications.
  -h, --help
      Show this help and exit.
  -v, --version
      Show version and exit.
  -V, --verbose
      Increase verbosity and show intermediate numbers used in caldat calculation.

Julian dates (abbreviated JD) are simply a continuous count of days and fractions since noon Universal Time on January 1, 4713 BC (on the Julian calendar).  Almost 2.5 million days have transpired since this date.Julian dates are widely used as time variables within astronomical software. Typically, a 64-bit floating point (double precision) variable can represent an epoch expressed as a Julian date to about 1 millisecond precision. Note that the time scale that is the basis for Julian dates is Universal Time, and that 0h UT corresponds to a Julian date fraction of 0.5.

Unix time, also known as Unix Epoch time is a system for describing a point in time. It is the number of seconds that have elapsed since 00:00:00 Thursday, 1 January 1970 UTC, minus leap seconds.  Every day is treated as if it contains exactly 86400 seconds, so leap seconds are to be subtracted since the epoch. The Unix epoch corresponds to the Julian day of 2440587.5

Examples:
Get julday on 26-Jan-1950 at 0:0:0:
    julday 01/26/1950 -t 0:0:0
        2433307.5
Convert above julian day back to calendar date:
    caldat 2433307.5
        1950-01-26T00:00:00Z
Get the day of year, the week day and week number for the above julian day:
    caldat 2433307.5 -f "%j %A %w"
        026 Thursday 4

Report bugs to <yaswant.pradhan>
EOF
}
# ----------------------------------------------------------------------------

while true; do
  case "$1" in
    -f |--format )  fmt="$2"; shift 2 ;;
    -h |--help )    usage; exit 0 ;;
    -v |--version ) version; exit 0 ;;
    -V |--verbose)  verb=1; shift 1 ;;
    -- )            shift; break ;;
    * )             break ;;
  esac
done

_julday="${1:-$(julday $(date -u +'%F %T') %f)}"
fmt=${fmt:-'%FT%TZ'}
JD_UNIX=2440587.5  # Unix epoch in days.

seconds=$(echo "$_julday" $JD_UNIX | awk '{printf "%f", ($1 - $2) * 86400.0}')
if (( verb )); then
  echo "Unix Julian Epoch = $JD_UNIX"
  echo "Input Julian day = $_julday"
  echo "Equivalent Unix seconds = $seconds"
  echo
fi

case "$OSTYPE" in
  darwin*) date -u -jf '%s' "$seconds" +"$fmt" 2>/dev/null;
    ;;
  linux*) date -u --date @"$seconds" +"$fmt";
    ;;
  *) echo "$OSTYPE not supported";
    ;;
esac