Bulk date conversion with GNU Date

GNU's version of the date command (which ships with almost all Linux flavors) can accept an arbitrary date and/or timestamp as input to display. Combined with the output formatting feature found in all posix flavors of the date command, you can use it to convert almost any format of date or time to a standard format.

axon ~$ date --date "yesterday"
Tue Oct 11 07:47:33 CDT 2011

axon ~$ date --date "Saturday, August 19, 1989"
Sat Aug 19 00:00:00 CDT 1989

axon ~$ date --date "21-APR-2001" +%Y-%m-%d
2001-04-21

So if you have a bunch of dates or timestamps, one per line, that you wish to convert:

axon ~$ cat file
1/21/2011
2/5/2011
2/10/2011
2/20/2011
3/7/2011
3/22/2011
4/16/2011
4/21/2011
4/26/2011
5/21/2011
1/24/2011
2/1/2011
3/6/2011
3/9/2011
May 25, 2011
6/23/2011
7/23/2011
8/10/2011
9-SEP-2011
Sat Sep 10 00:00:00 CDT 2011

... say to ISO 8601 Year-month-date format , you can knock it out easily like this:

axon ~$ cat file | while read line; do date --date "$line" +%Y-%m-%d; done
2011-01-21
2011-02-05
2011-02-10
2011-02-20
2011-03-07
2011-03-22
2011-04-16
2011-04-21
2011-04-26
2011-05-21
2011-01-24
2011-02-01
2011-03-06
2011-03-09
2011-05-25
2011-06-23
2011-07-23
2011-08-10
2011-09-09
2011-09-10