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
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