Contains the date and time when the object or subobject was last modified.
This property is initially set to the same value as the PR_CREATION_TIME (PidTagCreationTime) property. Attachment subobjects can update it as necessary by copying the last modification time maintained by the native file system. A client application can set this property until the first call to the IMAPIProp::SaveChanges method. From then on the provider should update PR_LAST_MODIFICATION_TIME during every IMAPIProp::SaveChanges call.