RRDtool

You want to fetch the 15 minute (900s/60s = 15 mins) average data for the last hour. You might try -r=resolution, -s start -1h(one hour from Now)

rrdtool fetch data.rrd  AVERAGE -r 900 -s -1h

Then convert time epoch time to human readable

[krit@mini tmp]$ date -d @1629298800
Wed 18 Aug 2021 10:00:00 PM +07