The namespace for EXSLT - Dates and Times is:

http://exslt.org/dates-and-times
Throughout this document, the prefix date is used to refer to this namespace. Any other prefix can be used within a particular stylesheet (though a prefix must be specified to enable the extension functions to be recognised as extensions).

To use these extensions, you need to declare this namespace as an extension namespace in your stylesheet. If your processor supports this module, then that's all you need to do, but if it doesn't, then you need to use a specific third-party implementation or the module stylesheet. Typically, your stylesheet will look like:

<xsl:stylesheet version="1.0"
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                xmlns:date="http://exslt.org/dates-and-times"
                extension-element-prefixes="date">

<xsl:import href="date.xsl" />

...
              
</xsl:stylesheet>

我测试文件夹如下:
├─ test.xml
├─test.xsl
├─date.xsl
├─elements
│  └─date-format
└─functions
    ├─add
    ├─add-duration
    ├─date
    ├─date-time
    ├─day-abbreviation
    ├─day-in-month
    ├─day-in-week
    ├─day-in-year
    ├─day-name
    ├─day-of-week-in-mo
    ├─difference
    ├─duration
    ├─format-date
    ├─hour-in-day
    ├─leap-year
    ├─minute-in-hour
    ├─month-abbreviatio
    ├─month-in-year
    ├─month-name
    ├─parse-date
    ├─second-in-minute
    ├─seconds
    ├─sum
    ├─time
    ├─week-in-month
    ├─week-in-year
    └─year