The LastModified property returns the date and time at which the object was last modified.
All Item objects
Date
Read-only
Applications can use the LastModified property to synchronize two schedules. For example, an application can compare this property on similar sets of objects in two copies of a schedule to determine which object is the latest version.
When synchronizing two different schedules, this property should be used instead of the ChangeNumber property. Change numbers are specific to a single schedule file, whereas this property uses dates that are independent of the schedule file.