Use event.date_published if using since to reconnect. This is the timestamp that our event platform maintains per event. When querying using the since parameter, a query is sent to the event platform asking to send events from this timestamp onwards.
Please note that the event platform has a retention period of 48 hours. So, if the query is for timestamp older than 48 hours, you will start receiving all the events with the oldest (48 hours old) event being sent out first.