Equipment maintenance automation with Assets AQL

[JuanC]

I have a global automation rule for equipment stored in Assets that creates a ticket when the date of the next service is less than 30 days. 

The part of the rule that is relevant is,

When an object is updated, For AQL - objectType = Equipment AND "next service - start date" < endOfMonth(30d)

This works when I test the AQL in Assets it filters the equipment that matches the query.

I am wondering if I can use the value of another attribute, in this case, "service frequency (days)" instead of endOfMonth(30d)?

If I enter this, it shows a green check mark but does not show the object:

objectType = Equipment AND "next service - start date" < "service frequency (days)" 

What am I doing wrong?

Schema: corp assets

Object Type: Equipment

Object:  sub-zero freezer

Attribute: next service - start date (type Date)

Attribute: service frequency (days)