In order to avoid race conditions, all timestamps manipulated by the server are not true HTTP date values, nor milliseconds EPOCH timestamps.
They are milliseconds EPOCH timestamps with the guarantee of a change per timestamp update. If two changes happen at the same millisecond, they will have two differents timestamps.
Last-Modified header with the last timestamp of the collection for a given
user will be given on collection and record GET endpoints.
marked_on will contain actual timestamps
(from device perspective), used for calendar year information display.
All timestamp of the app will be set in milliseconds.