Ads go live at 12am on their start date and run to 11:59pm on their end date.
The timezone these times go off of are set in your network settings.
If you want an ad to run for 1 day, you’ll want to set it to start on the day of and end the same day. So for example, an ad that should run on May 17th would start on May 17th and end on May 17th.