Yes, this is a tricky one. Details on the brevet card are set up automatically by the system based on the start time and control distances. There's no option to give the start time as a range, or to manually alter the timings on the brevet card (at least, I don't think there is - unless you list them all as info controls and put extra details in the info question field)
Just as a point of information on the practicalities - as Organiser you
can alter the intermediate (and finish) times as printed in the brevet card. Each pair of times is editable, or to be more accurate can be overridden with any text of your choosing, in the dialog where you set up the controls. It's not very obvious - it's not meant to be.**
So for example if your first control is a cafe at 57km the computed times would be 09:54 - 11:48 but if you know the cafe won't open until 10 o'clock you can (should) substitute the text 10:00 - 11:48.
If you wanted to allow a 1h start window you could even replace that with 10:00 - 11:48/12:48, and similarly for the other controls. Although unless a control is staffed by a volunteer I don't see any point in being anal about intermediate passing times (IMHO).
** there's no option to edit the Start Time (as displayed on the card) to indicate a 'start window' but if theres a demand for it (because of the 'new normal') I would be very happy after discussion with Ian H to add this. There's not really a lot of point as long as cards are handed out at the start anyway - there's already a box to write the actual time into. If you want to indicate in the Calendar that there is a range of start times, that can only currently be done in your event description texts (ie there's no data field for 'Start Closing Time' - and even if there was, the new website wouldn't know what to do with it).