Max/Min per item from a changing list
 

I am pulling in a DIR text file daily. This gives me
a=date, b=time, c=filesize, d=filename

each day there are a different number of entries, 10-40, some days none.

I want to calculate the thru-put of the file creations. On the 20th, the first file was made at 8am and the last was 5pm, sum of the file sizes is 1800. So 9 hours made 1800 = 200 per hour.

sumif(a:a,a2,b:b) gets me the filesize based on date.
How do I get the max/min time for a date to find my time difference complete the formula?

The goal is when I add more data tomorrow, I just copy the formula down or have it already there.