To change pick up times to a round number

My pick up time slots are every 15 mins…so when they check out it gives them 17.00-17.15, 17.15-17.30, 17.30-17.45 and so on… but some are selecting the wrong
Time slot ie 17.00-17.15
For a 17.15 collection, where as I guess the 17.15-17.30 is too late as they think it’s the window they can collect for, and then they end up leaving notes but messes with the limits per slots… hope that makes sense