Average daily values

Dear all,

I have a table of data covering the last 9 months based on values automatically collated from 15 minute intevals.
The date/time is in column A (01/01/2009 00:00) with the data collected in column D.

My wish is to get the average daily data from column D and I am slowly losing my head!!!

Is there anyway of getting a formula to auto-average the daily values bearing in mind there are currently 96 daily entries.

I have tried converting the first 5 digits of column A to numeric (i.e. 31894 for 01/01) then trying to write a formula saying =average(D1:D24577,if(range="31894",1)).

I can now see a simpler way but am so confused after an hour or so of trying.

Each day has 96 readings so I need an auto adding formula. average column cell A would say =average(D1:D96).
Is there are way to have the cell below auto-update itself to look at the next 96 values and so on and so forth?

Sorry if none of this makes sense!

Damien