Mod_date update technique

I have a column named mod_date on a particular table. It has a default
value of now() which is the create date of the row. However, I want to
update that field with the current timestamp whenever its record is
updated.

I know how to do this using PHP by including the date() in the data I'm
using to update the record. However, I would like to do this on the
PostgreSQL side of the transaction. I can't figure out how to write a
trigger or other technique to do this.

Simply asked: What is the easiest way to automatically update the mod_date
field when the record is updated?

Gary

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* Gary B. Hoffman, Computing Services Manager  e-mail: ghoffman@ucsd.edu *
* Graduate School of International Relations and Pacific Studies (IR/PS) *
* University of California, San Diego (UCSD)       voice: (858) 534-1989 *
* 9500 Gilman Dr.  MC 0519                           fax: (858) 534-3939 *
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