We ran into this bug as well.
We found out that lastModifiedDateworks when:
The event is created and modified only by the Desktop or Web Calendar Interface. When you change it on your iPhone, lastModifiedDate won't be updated by Desktop/Web any longer.
- Create event on Desktop -> lastModifiedDate is set
- Check event on iPad -> lastModifiedDate correct
- Modify event on Desktop and check event on iPad -> lastModifiedDate is updated
- Modify event on iPad and check event on Desktop -> lastModifiedDate is updated
- Modify event on Desktop and check event on iPad -> lastModifiedDate is NOT updated
OR
Desktop and Web Interface are not used. Tested with iPad Calendar App, GoogleMail and Exchange. Events can be modified on both ends of synchronisation.