grouping a datetime column only by time

I have a column datedate format, with 15 min intervals and another column called datavalues with corresponding data. I want to roll up the data values to 1 hr. I am attaching a screen shot. . So my new column should have the aggregated value of "DATA_VALUES" column. and the datetime column should represent just hours but not 15 min. Please help, I have tried cast() to convert to time but not able to proceed further.