Date Daily Discharge (cfs) Daily Discharge (m^3/s) 1/1/1967 2530 71.6416218778 1/2/1967 2530 71.6416218778 1/3/1967 2550 72.2079588096 1/4/1967 2550...

EXCEL!! I have 50 years of daily river flow data and I need to aggregate this into both monthly flow rate and yearly flows....I can't just add up the data by 30 rows each because some months are longer than other.  I know there has to be a way to do this in excel that can account for varying month lengths and leap years and whatnot.