[xsl] XPath string comparison of two strings representing calendar dates

Hi Folks,

I have an XML document that contains two elements with values representing 
calendar dates, in YYYYMMDD format, e.g.,

<Test>
    <Date1>20170816</Date1>
    <Date2>20170817</Date2>
</Test>

This is the format that iCalendar (RFC 5545) uses to represent dates.

I wish to test to see if the calendar date represented in <Date2> is greater 
than (occurs later in chronological time) the calendar date represented in 
<Date1>. Here's an XPath expression to do the test:

/Test/Date2 gt /Test/Date1

In this case, the XPath returns true.

Note that I am doing mere string comparison. I am comparing the string 
'20170817' against the string '20170816'.

Will the XPath always work? 

Is there a counterexample, where <Date2> holds a value representing a calendar 
date that is less than (occurs earlier in chronological time) the calendar date 
represented in <Data1>, and yet the XPath returns true? 

If there is a counterexample, what XPath expression will always work?

/Roger
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