Please attach The Workbook with Some fake data, as I Think Your question is bit Complex as You have Created a String Field and You can be helped through by a workbook only.
Not sure how you created this 15 mins group but basically you need to create a calculated field like:
Current_time>= [15min group start time] and Current_time<[15min group end time]
and put this on filter shelf and choose TRUE.
there in your filter go to top and tell him that you always want the maximum, that will always show you the last record of your group of 15 min