If you are very bored on a flight you can read this:
https://www.eurocontrol.int/sites/de...al-current.pdf
Procedure for Modifying the EOBT of a Flight not having received an ATFCM Slot
Procedure
- To amend the EOBT to a later time, a DLA or CHG message shall be sent to IFPS.
- To amend the EOBT to an earlier time, a CNL message shall be sent to IFPS, which, on receipt of ACK message, shall be followed by a new flight plan with the new EOBT indicated.
Note: The replacement flight plan procedure shall not be used.
5.1.4.1
Basically all flights in Europe are monitored by Eurocontrol to make sure no ATC sector gets overloaded, this is done through your EOBT +\- 15 min. If flights depart outside that the big plan does not work anymore so you need to refile with your correct EOBT so the realtime flow calculator knows if it will affect a sector down route.
Ready messages only apply to slots.