datenum() #@ Convert DATE to day NUMBER in C.E.
{         #@ USAGE: datenum [DATE [VAR]]
  local _y _m _d junk var num base=-306 ymd _dn
  local invalid="Invalid date" jg_m=175209 j2g_skip=11
  local j2g='Invalid date (Julian to Gregorian changeover)'

  [[ $1 == -- ]] && shift

  if (( $# == 0 )) || is_var "$1"
  then
    today ymd
  elif is_date "$1"
  then
    ymd=$1
    is_var "$2"
  else
    err 1 "$invalid: $1" || return
  fi
  read _y _m _d junk <<< "${ymd//[!0-9]/ }"
  if [[ $_y == [0-9][0-9][0-9][0-9][0-1][0-9][0-3][0-9] ]]
  then
    _d=${_y: -2}
    _m=${_y:4:2}
    _y=${_y:0:4}
  fi
  _d=${_d#0}
  _m=${_m#0}
  _m=$((12 * _y + _m - 3))      ## Calculate number of months from March
  _y=$(( _m / 12))              ## and adjust year if necessary
  _ly=$(( _y/4-_y/100+_y/400 )) ## number of leap days
  _dn=$(( (734 * _m + 15) / 24 -  2 * _y + _ly + _d + base ))
  if [[ $_dn -ge 639786 && $_dn -le 639795 ]]
  then ## date was swallowed by Julian to Gregorian changeover
    err 1 "$j2g" || return
  elif [[ $_dn -lt 1 ]]
  then
    err 2 "$invalid $ymd" || return
  fi
  (( _dn > 639796 )) && _dn=$(( _dn - j2g_skip ))
  [[ $var ]] && printf -v "$var" %d "$_dn" || printf '%s\n' "$_dn"
}