Comparing dates and ignoring time zone

I'd like to compare two dates for equality, where the time zone of one
or other is not considered.

The first date is:

$first = '2006-03-05'

..and the date to compare might be:

$second='2006-03-05T20:00:00.0000000+01:00'

Or

$second='2006-03-05T22:19:43.000Z'

I've tried the following, but it doesn't produce equality:

<xsl:if test="$first = xs:date(xs:dateTime($second))">
...
</xsl:if>

Any ideas?

Thanks, David