Using Saxon 9 (not schema aware), is there anyway to test if a value is in xs:dateTime format?

for exmaple, XML input:

<time>2006-05-04T18:13:51</time>

XSL:

<xsl:if test="time != xs:dateTime">
  <xsl:message terminate="yes">error msg</xsl:message>
</xsl:if>

I am also looking to test if an optional element is contained within the input XML? Is there a best practices for this?

<xsl:if test="time">

>is there anyway to test if a value is in xs:dateTime format?

$value castable as xs:dateTime

>I am also looking to test if an optional element is contained within the input XML?

<xsl:if test="x">

where x is a path expression that will select the element if it's present.

Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference