1minute data to average of every 30minutes

[boneout14]

I have 1000's of lines of data collected every minute and I need to average this to every 30 minutes. I use a formula to do this to average every hour but can't seem to get my brain to work right to get this for the 30 minute average.

The hourly average formula is =AVERAGE(OFFSET(B$8,(ROWS(C$8:C8)-1)*60,,60))

This is the formula I used to change the date time column to every 30 mins =A8+0.5/24

Please Help!!!

Cheers