# Tue Jan  6 16:54:02 EST 2004
# NAME: days-between
# Copyright 2004, Chris F.A. Johnson
# Released under the terms of the GNU General Public License

DMY=${DMY:-123}

dmy2jd()
{
    parse_date $*
    check_date || usage "???"
    ## forumla from <http://www.hermetic.ch/cal_stud/jdn.htm#comp>
    jd=$(( ( 1461 * ( $y + 4800 + ( $m - 14 ) / 12 ) ) / 4 +
	    ( 367 * ( $m - 2 - 12 * ( ( $m - 14 ) / 12 ) ) ) / 12 -
	    ( 3 * ( ( $y + 4900 + ( $m - 14 ) / 12 ) / 100 ) ) / 4 +
	    $d - 32075 ))

}

usage() {
  echo "Usage: ${0##*/} [D]D/[M]M/YYYY [D]D/[M]M/[-]YYYY"
  echo "       $*"
  exit 5
}

check_date() {
    days_in_month &&
    [ $y -ge -4800 ] &&
    [ $d -ge 1 ] &&
    [ $d -le $_DAYS_IN_MONTH ]
}

days_in_month() {
    case ${m#0} in
        9|4|6|11) _DAYS_IN_MONTH=30 ;; ## 30 days hath September...
        1|3|5|7|8|10|12) _DAYS_IN_MONTH=31 ;;
        2) is_leap_year ${y:-`date +%Y`} && _DAYS_IN_MONTH=29 || _DAYS_IN_MONTH=28 ;;
	*) _DAYS_IN_MONTH=0
	   return 1 ;;
    esac
}

is_leap_year() {
    y=${1:-`date +%Y`}
    case $y in
        *0[48] |\
        *[2468][048] |\
        *[13579][26] |\
        *[13579][26]0|\
        *[2468][048]00 |\
        *[13579][26]00 ) _IS_LEAP_YEAR=1
                         return 0 ;;
        *) _IS_LEAP_YEAR=0
           return 1 ;;
    esac
}

parse_date()
{
  dt="$*"
  f1=${dt%%[!0-9]*}
  f2=${dt%[!0-9]*}
  f2=${f2#*[!0-9]}
  f3=${dt##*[!0-9]}
  case ${DMY:=123} in
      123|dmy) d=${f1#0}; m=${f2#0}; y=$f3 ;;
      213|mdy) d=${f2#0}; m=${f1#0}; y=$f3 ;;
      321|ymd) d=${f3#0}; m=${f2#0}; y=$f1 ;;
      312|myd) d=${f3#0}; m=${f1#0}; y=$f2 ;;
      132|dym) d=${f1#0}; m=${f3#0}; y=$f2 ;;
      231|ydm) d=${f2#0}; m=${f3#0}; y=$f1 ;;
      *) return 1 ;;
  esac
}

[ $# -ne 2 ] && usage "Requires 2 arguments"
dmy2jd $1 # ${1//[!0-9-]/ }
jd1=$jd
dmy2jd $2 #${2//[!0-9-]/ }
diff=$(( $jd - $jd1 ))
echo ${diff#-}