When you set up return options, you're given the choice of how many days you'd like to allow them to be able to make their return.
When choosing how many days to offer, you'll want to know how they get counted, and when the days start.
Let's say you have a 15 day return policy, and someone buys a product on the 1st of the month.
No matter when they buy the product on the 1st, they would have until 11:59pm on the 15th to be able to return their product, 15 days after their purchase.
So when you pick how many days for your return policy, make sure to take into consideration fulfillment and shipping times, and make it clear that they until the end of the last day to make their return.