You don't provide enough data to do any testing, Dave, but I would approach this by using a function that converts the date and time elements into the dateTime value of the start of the 3-hour period.

Then you have a single string with which to group.

I hope this is helpful.

At 02/01/2026 13:58 +0000, you wrote:
>xslt 2.0
>
>Input
><data>
> 
>   <period>
>      <pwr>0.136000</pwr>
>      <cost>0.95204760000</cost>
>      <start>
>         <date>12-01</date>
>         <time>00:00</time>
>      </start>
>      <end>00:00</end>
>   </period>
>   <period>
>      <pwr>0.171000</pwr>
>      <cost>1.19705985000</cost>
>      <start>
>         <date>12-01</date>
>         <time>00:30</time>
>      </start>
>      <end>00:30</end>
>   </period>
>   <period>
>      <pwr>0.138000</pwr>
>      <cost>0.96604830000</cost>
>      <start>
>         <date>12-01</date>
>         <time>01:00</time>
>      </start>
>      <end>01:00</end>
>   </period>
>
>I've a months worth of data, ea 30 mins.
>How to group by and sum 3 hour blocks within
>one day please?
>group starting with  date AND time AND
>group ending with date AND time seems a logical
>extension to string operations?
>  
>but that's not available?
>
>Suggestions please?
>
>
>-- 
>Dave Pawson
>XSLT XSL-FO FAQ.
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