RFC 5545, "Internet Calendaring and Scheduling Core Object Specification (iCalendar)", September 2009Source of RFC: calsify (app)
Errata ID: 5082
Reported By: George Sexton
Date Reported: 2017-08-10
Section 22.214.171.124 says:
It should say:
The value must not be in the future.
If future values are allowed for LAST-MODIFIED, it makes it very difficult for software to perform change management for a VEVENT.
Say for example, an event is imported from a source and the VEVENT has a future date. For example, December 20 2017 12:00:00Z
That event is then edited and assigned the actual correct date and time. Say August 9 2017 13:45:44Z.
Software that examines the VEVENT's LAST-MODIFIED to determine if the VEVENT record has been modified will not see the VEVENT is updated.
In order for different systems to perform change management, the value for LAST-MODIFIED should NEVER be a future date.