= 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
{{{
#!sh
[krit@mini tmp]$ date -d @1629298800
Wed 18 Aug 2021 10:00:00 PM +07
}}}

Convert date from epoch time to human readable format
{{{
#!sh
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)