datetime grouping problem in XSLT

I can't seem to get my head around this, any help would be greatly appreciated: I have a list of datetime nodes, grouped by day (for-each-group),

<xsl:for-each-group select="response/result/doc" 
group-by="functx:day-of-week(xs:dateTime(./date[@name='ds_startshow']))">

This works fine, the datetimes are grouped by day of the week. The problem is that I need to include some datetime nodes of the next day to the current day. For example: I need to include saturday 0:15 to the friday "group" (the early hours belong to the previous day). So a single day stretches out to some some hours of the next morning. Basically, a new day starts at 2:00 instead of 0:00. How would I accomplish a grouping like that with XSLT(2). Presumably I need to write a fun开发者_StackOverflow社区ction to tackle this? Do I need to iterate over the days? Not sure about the best practice in this particular case.

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What you want to do is take the actual time and subtract two hours, thus creating a virtual day boundary at 02:00, and then group by that. This is untested, but you might try:

$x - xs:dayTimeDuration("PT2H")

where $x is the actual time. The result is the actual time minus 2 hours, which you can then use to group by.

You might look at this site, which I found by googling for "xslt date arithmetic", and is where I found the syntax for xs:dayTimeDuration()