saxon:in-summer-time($date as xs:dateTime, $region as xs:string) ==> boolean?
The $region argument may either be an ISO two-letter country code (for example "de" or "es"), or
an Olsen timezone name (such as "America/New_York" or "Europe/Lisbon"). The function returns true if the given
date/time is in summer time (daylight savings time) in that country or timezone, as far as can be determined
from the Java timezone database. If the information is not available the function either returns false or an empty sequence.
If a country code is specified for a country that spans different timezones with different daylight savings rules (for example, the US), then one timezone in that country is chosen arbitrarily.
For reliable results, the supplied date/time should include a timezone. This does not need to correspond to
the timezone named in the second argument; it is there purely to ensure that $date represents a single
point in time unambigously.
