Interpret DTSTAMP as LAST-MODIFIED for not-scheduled objects
The List perspective of the Memo, Task and Calendar windows has a “Last modified” column, which probably maps to the Last-Modified iCalendar property.
In the absence of a Last-Modified property, the value for the Last Modified column shall be taken from the DTSTAMP property.
https://datatracker.ietf.org/doc/html/rfc5545#section-3.8.7.2 says about DTSTAMP:
Purpose: In the case of an iCalendar object that specifies a "METHOD" property, this property specifies the date and time that the instance of the iCalendar object was created. In the case of an iCalendar object that doesn't specify a "METHOD" property, this property specifies the date and time that the information associated with the calendar component was last revised in the calendar store.