1. Create Maintenance Expiry Date field under Asset Tab

2. Create Email Notification using Maintenance Expiry Date field

3. Select "Maintenance Expiry < DATE_ADD(CURDATE(), INTERVAL 30 DAY)" for notification of maintenance expiry in 30 days

On clicking "Create Notification",  Reporting Email Alerts page will be displayed

Select the email alerts and select the run frequency, hourly, daily, weekly or monthly.

You will notice that apostrophe has been added in the front and back of the string of characters you supplied earlier, "DATE_ADD(CURDATE(), INTERVAL 30 DAY)",  Remove the 2 apostrophes as they turn the date into a character string that invalidates arithmetical comparison