Problem
When running an ad campaign, Roblox requires that campaigns be at least 24hours in duration, meaning that you cannot have a campaign that is only contained within 1 day (see below):
This causes issues when you are trying to run a 1-day campaign with a set daily budget and only be billed for 1 day, because the ads system will automatically bill you for 2, even if your period is 24hrs.
What Happened
In the example below, I ran a 24-hour campaign with a 5 ad credit daily budget, expecting to be charged 5 ad credits for the single day-long period that it ran.
Campaign Tile (5 ad credit daily budget)
Campaign Period (24hr window)
Amount Billed (10 ad credits [x2 daily budget])
What I expected to happen
When setting up a campaign with the minimum 24hr duration and a set daily budget, I would expect to be billed for 1 day (24hrs) as I selected rather than 2 whole days.