#!/bin/sh

# $1 = 2013-11-08

date_in=2013-11-08
len=28

len_s=$(( 28 * 24 * 60 * 60 ))

# convert to ts
date_s_in=$(date -jf %Y-%m-%d +%s $date_in)

# add len
date_s_out1=$(( date_s_in + len_s ))
date_s_out2=$(( date_s_out1 + len_s ))
date_s_out3=$(( date_s_out2 + len_s ))

date_out1=$(date -jf %s +%Y-%m-%d $date_s_out1)
date_out2=$(date -jf %s +%Y-%m-%d $date_s_out2)
date_out3=$(date -jf %s +%Y-%m-%d $date_s_out3)

echo "$date_in + $len days = $date_out1"
echo "$date_out1 + $len days = $date_out2"
echo "$date_out2 + $len days = $date_out3"