From
http://linuxconfig.org/addition-and-subtraction-arithmetics-with-linux-date-command
Addition and subtraction arithmetics with Linux date command
This article features some simple examples on how to add or subtract a time from a current date. At first we use a native date sytax to do this trick and later we will see how this can be done manually by converting date to an epoch time. Let's start with a simple backup script based on the date Linux command:
#!/bin/bash tar cjf linuxconfig_$(date +%H%M-%d%m%Y).tar.bz2 ~/public_html
Every time the script is executed it will create a file with a current date included in the file name. The backup problem is solved. However, we do not usually want to keep all backup files indefinitely or until they consume all available free space. This is where the subtraction arithmetics with Linux date command comes handy. Let's see couple examples how to subtract time from a current date using date string: Subtract 10 years from a current date:
$ date Mon Mar 7 09:26:13 EST 2011 $ date --date="10 years ago" +%H%M-%d%m%Y 0926-07032001
Subtract 3 months from a current date:
$ date
Mon Mar 7 09:29:39 EST 2011
$ date --date="3 months ago" +%H%M-%d%m%Y
0929-07122010
Subtract 255 days from a current date:
$ date Mon Mar 7 09:31:13 EST 2011 $ date --date="255 days ago" +%H%M-%d%m%Y 0831-25062010
Subtract 32 weeks from a current date:
date; date --date="32 weeks ago" +%H%M-%d%m%Y Mon Mar 7 10:05:34 EST 2011 0905-26072010
Subtract hours minutes from a current date:
$ date; date --date="5 hours ago" +%H%M-%d%m%Y Mon Mar 7 09:54:42 EST 2011 0454-07032011
and as you have already guessed we follow the same format to subtract minutes from current date:
$ date; date --date="5 minutes ago" +%H%M-%d%m%Y Mon Mar 7 09:32:48 EST 2011 0927-07032011
We can now improve our simple backup script to keep only files which are not older than 6 months:
#!/bin/bash tar cjf linuxconfig_$(date +%H%M-%d%m%Y).tar.bz2 ~/public_html rm linuxconfig_$(date --date="6 months ago" +%H%M-%d%m%Y).tar.bz2
Going into the future with date command is as easy as going into the past. All what needs to be done is to add "-" ( minus ) sign in front of every date string. For example you can ask date command to add 12 hours to a current date and time:
date; date --date="-12 hours ago" +%H%M-%d%m%Y Mon Mar 7 10:02:17 EST 2011 2202-07032011
On some Unix systems the date syntax described above may not be available. In this case here as a simple example on how to the do all above using epoch time. epoch time is simply a number of seconds since "Jan 1, 1970 00:00:00". Therefore, epoch time "1" using universal time is:
$ date -ud@1 Thu Jan 1 00:00:01 UTC 1970
Lets see how we can subtract 2 weeks 3 days and 23 seconds from a current date sing epoch time. Please not that we use only universal time for a following calculations. In the first step we need to convert 2 weeks 3 days and 23 seconds into seconds. Use a following table to guide you through this process:
| 1 year ( 365 ) | 31,536,000 seconds |
| 1 week | 604 800 seconds |
| 1 day | 86 400 seconds |
| 1 hour | 3600 seconds |
| 1 minute | 60 seconds |
| 1 second | 1 second |
2 weeks 3 days and 23 seconds = 2 x 604 800 + 3 x 86 400 + 23 2 weeks 3 days and 23 seconds = 1 209 600 + 259 200 + 23 2 weeks 3 days and 23 seconds = 1 468 823 seconds
Now, that we have number of seconds lets subtract this number from a current epoch time:
$ date;echo `date --universal +%s` - 1468823 | bc Mon Mar 7 10:43:14 EST 2011 1297986171
All what has left is to convert output echo time to universal human readable date format:
$ date -ud@1297986171 Thu Feb 17 23:42:51 UTC 2011
To add 2 weeks 3 days and 23 seconds use the same process but use addition instead of subtraction:
$ date;echo `date --universal +%s` + 1468823 | bc Mon Mar 7 11:02:29 EST 2011 1300924972
Convert epoch time:
$ date -ud@1300924972 Thu Mar 24 00:02:52 UTC 2011