Count uniques?

I need to know how I use the count function to have a sum of days
ie: I have several transactions in several days like hmmm Tasks
Task -> id, price, day , description

so in every day I did some tasks
I sum them with the Task.sum(:price)
but I want to do an automatized way to have an average description (sum
/ days )
how can I do it ?
ps: day is datetime …

You can substract datetimes to get the difference between them in
seconds. -
Use that to average?