Regarding getting Date - n days Date

Question

Suppose I have a particular date stored in a variable date_m. I want ((date_m)-25) date.

For example: I have 15/09/2014 stored in my variable , then I want 21/08/2014 returned if I subtract 25 from the date stored in variable.

Answer 1

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

Answer 2

In the following the date and number of days are declared.

The script turns the dates in seconds and computes what timepoint 1 (Tp1) is if the number of days (in seconds) is subtracted from the given timepoint 2 (Tp2). In the end seconds is converted back in date.

Date_m=2014/09/15
Days=25
Seconds=$(echo "$Days"*60*60*24| bc -l)
Tp2date=$(date -d "$Date_m" +%Y/%m/%d)
Tp2sec=$(date -d "$TD" +%s)
Tp1sec=$(echo "$TDsec"-"$Seconds"| bc -l)
Tp1date=$(date -d @$Tp1sec)
echo "$Tp1date"

Answer 3

Somewhat belatedly, here is one way to handle non-US dd/mm/yyyy format dates with the 25 day relative adjustment:

date_m='15/09/2014'
date --date "$(IFS=/ read d m y <<<"$date_m"; echo "$m/$d/$y - 25 days")" +%d/%m/%Y

21/08/2014

It does assume a shell such as bash that's capable of handling a string redirection <<<, and GNU date.