J-1 return date from authorized absence: day N or day N+1?

Question

I am a J-1 Research Scholar currently outside the US on a temporary absence, which was approved (and hence recorded in SEVIS) through a specific date, e.g. April 15. I am currently looking at return flight options. When should I re-enter the US: on the last day of my allowed absence, i.e. April 15, or on the first day on which I am supposed to be back, i.e. April 16? The question seems so silly that I don't want to bother the international office with this. Would the former be considered inappropriately returning early, or would the latter be considered inappropriately returning late?