I am aware that the Direct Message limit of 250 per day is a limit on the user not an API rate limit however while writing a particular DM-focused app it is important that we understand “who’s day”?
Is it a calendar day and if so is it GMT, the servers time zone, or the sending user’s time zone?
Or is it, as some seem to think based on anecdotally observed behaviors, a sliding 24-hour window?
We could start slamming out DM’s until we hit limits and try to figure it out but it seems somewhat more straightforward and efficient to just ask 
Anyone actually know ?