SELECT Records using multiple ciritria

[Gary Ehrenfeld]

I have a diabetes database that the user's enter in their blood glucose readings that they take during the day.

Date-------Time---Value
10/15/16--08:35--120
10/15/16--14:00-- 85
10/17/16--22:00--146
10/21/16--04:45--256
10/21/16--12:00-- 95
and so on.

I need to average the values for the last 7, 14, and 30 days of readings from the last reading date.

How can I do one select statement where I get the average from all values from the last date entered back 7, 14,30 days