How do start_date and end_date work for schedule.vacations

If I have the following query:

{
"module":"schedule.vacations",
"method":"GET",
"request":{
"start_date":"May 10, 2017",
"end_date":"May 15, 2017",
"mode":"manage"
}
}

How do the start_date and end_date parameters work? Am I saying get me all vacations that have a:
start date = May 10, 2017 AND an end date = May 15, 2017?
start date after May 10, 2017 and an end date before May 15, 2017?
start date OR end date in between May 10, 2017 and May 15, 2017?
something else?

Based on the example in the documentation, it seems that the answer is the third option, however,
the responses I'm getting from my instance of Humanity based on this understanding don't always match up with what I see in Humanity. I'm not sure if timezones are throwing off the results or not, but it would help to be clear on exactly how the start_date and end_date parameters work.