#!/bin/sh

# This shell script determines from date(1) what today's date is, then will
# output the date in the format yymmdd.  If an offset is specified using the 
# "-o" option, it will determine the date of the day corresponding to the 
# current date + or - the offset (using the sign supplied by the user).
# An example follows:
#
#	day -o 4
#
# will provide the date four days in the future.
#
# 	day -o -27
#
# will provide the date 27 days ago.
#
# The algorithm provided will work across months and years, and will determine
# leap years.
#
#
# Author:       Hugh Mahon                              |__|     
#                                                       |  |     
#                                                          |\  /|     
#               e-mail: hugh@mahon.cwx.net                 | \/ |
#                                                        

#set -x

function year_len
{
	let leap=0
	let daysInYear=365

	if (( $year % 400 == 0 ))
	then
		let daysInYear=366
		let leap=1
	elif (( $year % 4 == 0 )) && (( $year % 100 != 0))
	then
		let daysInYear=366
		let leap=1
	fi


}


let offset=0

while [ $# -gt 0 ]
do
	case $1 in
		-o)	offset=$2
			shift;;
		*)	echo "usage: $0 [-o #]"
			echo "          where -o # specifies an offset from today's date (+/-)"
			exit;;
	esac
	shift
done

today_date=`date +%j | sed -e s/^00// | sed -e s/^0//`

let today_date=today_date+$offset

year=`date +%Y`

leap=0
let daysInYear=365

year_len

if (( today_date < 1 ))
then
	while (( today_date < 1 ))
	do
		let year=year-1
		year_len
		let today_date=today_date+daysInYear
	done
fi

if (( today_date > daysInYear ))
then
	while (( today_date > daysInYear ))
	do
		let today_date=today_date-daysInYear
		let year=year+1
		year_len
	done
fi

let i=1
let days=0
let this_month=0
let days_so_far=0
while [ $i -lt 13 -a $this_month -eq 0 ]
do
	if [ $i = 1 -o $i = 3 -o $i = 5 -o $i = 7 -o $i = 8 -o $i = 10 -o $i = 12 ]
	then
		let days=days+31
	elif [ $i = 4 -o $i = 6 -o $i = 9 -o $i = 11 ]
	then
		let days=days+30
	elif [ $i = 2 -a $leap -eq 1 ]
	then
		let days=days+29
	elif [ $i = 2 ]
	then
		let days=days+28
	fi

	if (( days < today_date ))
	then
		let days_so_far=days
	else
		let this_month=i
	fi

	let i=i+1

done


let day=today_date-days_so_far

DATE="$year"

if (( this_month < 10 ))
then
	DATE="${DATE}-0$this_month"
else
	DATE="$DATE-$this_month"
fi

if (( day < 10 ))
then
	DATE="${DATE}-0$day"
else
	DATE="$DATE-$day"
fi

echo $DATE