It seems that when routing using an idealized arrival time (day-of-week + time-of-day), if the wallclock time is too close to the target arrival time, the routing request fails with the error "Unable to calculate route. Start time occurs in the past."
(select dateType and localTime to just after the current time)
It seems that this should be able to shift the target arrival time forward a week and successfully route, as is the case if you specify an arrival time a minute in the past. Are there any options that will force it to do so?