Hi,
you can do it with a simple shell script (on unix).
use the date command (date +%d-%m-%Y) to get the present date to "extn" variable. Append it to the filename backup.${extn}.ldif (like backup.04-27-2007.ldif). Use this filename in the export command db2ldif.
db2ldif -n userroot -a /Dir/backup.${extn}.ldif
Hope this helps. Please make sure to purge old files else your FS would be full one day.
Regards
Randip