How do you run an XPath contains() on a collection?

I have an XML document whose structure is similar to the following:

<list>
  <item>
    <delivery>
      <date>2014-12-31T00:00:00.000Z</date>
      <!-- ... -->
    </delivery>
    <!-- ... -->
  </item>
  <item>
    <delivery>
      <date>2014-12-31T01:10:11.000Z</date>
    </delivery>
  </item>
  <item>
    <delivery>
      <date>2014-12-31T12:30:15.059Z</date>
    </delivery>
  </item>
</list>

What I want to do is check the deliveries are all on the same day. What I did in SoapUI was check the following is true:

contains(/list/item[*]/delivery/date, "2014-12-31")

SoapUI then complains you can't hand contains() a collection. Fair enough. I then tried

/list/item[*]/delivery/date[contains(., "2014-12-31")]

but that seems to return the first date in its entirety: 2014-12-31T00:00:00.000Z

So, how can I run a contains on my collection? Alternatively, how can I check the days are the same for all deliveries?