To convert 15-minute timestep data to hourly average, follow these steps:

1. Insert a new column next to the data column. In this new column, enter the formula to round the datetime to the nearest hour. For example, if your datetime is in cell A2, enter the formula
   Code:

   "=ROUND(A2*24,0)/24"

   in the new column B2. This will give you the datetime at the top of the hour.
2. In the fourth column, enter the formula to calculate the hourly average. For example, if your data is in column C and your datetime rounded to the nearest hour is in column B, enter the formula
   Code:

   "=AVERAGEIF(B:B, B2, C:C)"

   in the fourth column D2. This will give you the average of the four 15-minute data points for the hour.
3. Copy the formulas in columns B and D down to all the rows of data.
4. If there is missing data, the AVERAGEIF function will ignore the empty cells and only calculate the average of the available data points.

That's it! You now have a third column with the datetime at the top of the hour and a fourth column with the hourly average.