#!/bin/sh
# juldif
# calculates the days difference between two dates and reports 
# the number days as jul1 - jul2 
# usage juldif jul1 jul2
# where julian date is in the form yyyyddd

 usage () {
 echo "Usage:"
 echo " juldif jul1 jul2"
 echo ""
 echo " Calculates the day difference between"
 echo " two julian dates (jul1 -jul2)"
 echo " where a julian date is in the form of yyyyddd."
	 }

if [ $# != 2 ]
then
usage
exit
fi

# This process subtracts arg2 from arg1. If arg2 is larger
# then reverse the arguments. The calculations are done, and
# then the sign is reversed
if [ `expr $1 \< $2` = 1 ]
then
jul1=$2
jul2=$1
else
jul1=$1
jul2=$2
fi

# Break the dates in to year and day portions
yyyy1=`expr $jul1 / 1000`
yyyy2=`expr $jul2 / 1000`
ddd1=`expr $jul1 % 1000`
ddd2=`expr $jul2 % 1000`

# Subtract days
res=`expr $ddd1 - $ddd2`

# Then add days in year until year2 matches year1
while [ `expr $yyyy2 \< $yyyy1` = 1 ]
do
diy=`yeardays $yyyy2`
res=`expr $res + $diy`
yyyy2=`expr $yyyy2 + 1`
done

# if argument 2 was larger than argument 1 then 
# the arguments were reversed before calculating
# adjust by reversing the sign
if [ `expr $1 \< $2` = 1 ]
then
res=`expr $res \* -1`
fi

# and output the results
echo $res