Implicit timezone global; must be initialized
[2] XPath and XQuery Functions and Operators 3.1 (2014-12-18 06:45:18 +09:00) <https://www.w3.org/TR/xpath-functions-31/#func-implicit-timezone>
The implicit timezone is implementation-defined.
The implicit timezone is implementation-definedXP.
[5] XPath and XQuery Functions and Operators 3.1 (2017-03-21 16:02:06 +09:00) <https://www.w3.org/TR/2017/REC-xpath-functions-31-20170321/#func-implicit-timezone>