They are frequently defined as either a place, e.g. crew room, or more often a time before scheduled departure is used. The operator has a responsibility to define the FTL reporting time taking into account of factors such as whether the crew room is airside or not, security arrangements or where the briefing takes place etc. If the reporting time is not defined, or you believe it does ot take into account reasonable factors, then get in touch with your aviation authority inspector (or anonymous report) asking for their input.