From Matchbook's rules:
• All open offers are automatically cancelled by Matchbook at the start of an event. Should a market remain open for trading past the start of the event due to an erroneously listed starting time, all trades made after the event's actual starting time will be voided.
• Clients are reminded that starting times posted on the site are estimates, as provided to us through publicly available sources. Events often start several minutes later than the official time; and where possible (due to TV coverage, for example), we will attempt to keep markets open until the event's actual starting time.