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

Convert date from epoch time to human readable format

OMD[monitor@501264b41d2d]:~/var/log$ awk -F"[][]" '{cmd="date -d@" $2;cmd |getline $2; close(cmd)}1' naemon.log 

Here we set a shell command date -d@$2 to a variable called cmd. Then with cmd |getline $2 the awk reads a line and pass second field value to the cmd we open it, then we need close the command we opened via pipe, so we used close(cmd)
Or we can used

OMD[monitor@501264b41d2d]:~/var/log$ tail naemon.log | awk -F"[][]" '{cmd="date -d@" $2;cmd |getline $2; close(cmd)}1'