Hi Adam,
That’s very strange. The route “AAA LDN ZONE 7-9” makes the ticket a day travelcard valid throughout zones 1-9. Normally the off-peak version of that is only valid after 0930. It’s possible that 0915 is a local easement for Watford High Street. I don’t think the gates at Chalfont will accept it before 0930.
Beyond that there is no need to go to Watford High Street at any time. It’s just a 1-9 travelcard.