How to Count as a 1 day if the dates are same in XSLT

You could:

• use xs:date() to evaluate those values as xs:date
• then subtract one date from the other to get the difference as xs:dateTimeDuration
• then use days-from-duration() to return that number of days (which would be zero)
• apply abs(), such that we don't have to worry about which date was on the left or right hand side and positive or negative values
• and then add 1 to that value

This stylesheet:

<xsl:stylesheet version="2.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xsl:template match="/">
    <xsl:text>Days: </xsl:text>
    <xsl:value-of select="abs(days-from-duration(xs:date(/*/*:Date[1]) - xs:date(/*/*:Date[2]))) + 1"/>
  </xsl:template>
</xsl:stylesheet>

Applied to this XML file:

<wd:example xmlns:wd="wd">
    <wd:Date>2021-08-18</wd:Date> 
    <wd:Date>2021-08-18</wd:Date>    
</wd:example>

Produces this output:

Days: 1