You're confused about language dialects.
XSLT 1.0 was defined by W3C in 1999. During its development it was often referred to as XSL, and this name survives in the conventional prefix xsl:stylesheet and the conventional file extension style.xsl.
Before XSLT 1.0 came out, Microsoft produced a product based on a working draft of the spec, with many of their own extensions. Older Microsoft documents refer to this obsolete dialect as XSL, but most people call it WD-xsl, after the namespace used to identify it. THis dialect is essentially dead.
XSLT 2.0 is still in draft, and not supported by any Microsoft products. It includes functions such as format-dateTime().
XSLT 1.0 allows the function library to be extended in a vendor-specific namespace. The MSXML4 product includes some such extensions, specifically for date and time formatting.
There are also some extension functions for date and time handling in the EXSLT library, see www.exslt.org
Hope this clarifies!
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference