UNIX for Advanced & Expert Users

#!/bin/bash

#call with deltaday 'date' 'deltadays'

#there is no error checking, use at your own use...

if [ ! -n "$1" ]
then
  echo "Usage: `basename $0` YYYYMMDD [+-days]"
  exit $E_BADARGS
fi  

if [ ! -n "$2" ]
then
  	deltadays=-1
  else
	deltadays=$2
fi  


epoch=$(date -j -f "%Y%m%d" $1 "+%s")

(( deltadays *= 60*60*24 ))
(( epoch += deltadays )) 

newday=$(date -j -f "%s" $epoch "+%Y%m%d")

echo $newday

date -d "yesterday"