Bash – determine if date is beyond 90 days in bash

You can just use the -d switch and provide a date to be calculated

date
Sun Sep 23 08:19:56 BST 2012
NEW_expration_DATE=$(date -d "+10 days")
echo $NEW_expration_DATE
Wed Oct 3 08:12:33 BST 2012 

  -d, --date=STRING
          display time described by STRING, not ‘now’

This is quite a powerful tool as you can do things like

date -d "Sun Sep 11 07:59:16 IST 2012+10 days"
Fri Sep 21 03:29:16 BST 2012

or

TZ=IST date -d "Sun Sep 11 07:59:16 IST 2012+10 days"
Fri Sep 21 07:59:16 IST 2012

or

prog_end_date=`date '+%C%y%m%d' -d "$end_date+10 days"`

So if $end_date=20131001 then $prog_end_date=20131011

With the GNU implementation of date, to display yesterday's date, enter:

$ date --date="1 days ago"

OR

$ date --date="-1 day"

For your question:

$ date --date="25 days ago"

OR

$ date --date="-25 day"

For using it with variables, you can use $():

pastDate=$(date --date="-25 day")
echo "$pastDate"

For general case n days and for a specific date:

#!/bin/bash
date1="Tue Sep 2 07:53:47 EEST 2014"
echo "Before? "
read n
date --date="$date1 -$n day"

Source