Grouping timestamp Data into 15 min interval with respect date

Hi, i have a timestamp data of 1 min interval. i want to convert one min data interval into 15 min interval data with respect to date. i have converted the data into table with three different columns. i have attached the sample file

first coulmn = date

second column = time

thrid column = data

the output table should be

Date Time Readings

29-Jan-2019 00:15:00 0

29-Jan-2019 00:30:00 0

29-Jan-2019 00:45:00 0

29-Jan-2019 01:00:00 0

so on

Also the data in the third column should sum the reading for 15 min and then for next 15 min and if there is less than 15 data points (i.e. less than 15 min data) then sum the avalible data points and time (e.g. 14 data points)

Looking forward for your help

Thanks