Dear Experts

I have created a log file of Winodows usages. When everytime Windows starts, a new recrod is added into table with date and time such as

1 21/05/2004 10:00
2 22/05/2004 08:30
3 24/05/2004 07:00

This works fine, but I want to keep records only of last 30 days.For example if records are more than 30 days or dates,the last 30 days records must remain and other records, from the beginning of the table, should automatically be deleted as a sequence.

How is it possible?
Please help