The format Excel uses for the xml-export depends on the added XML-schema. This can be explicitly created and added or -- as I suspect in your case -- has been implicitly/ automatically generated by Excel.
In my example I have two colums, the first one has via XML-Schema to format date-time, the second is normal text and only for the representation in Excel (column format) a date.

When I export this as XML I get two seemingly completely different results:
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<dataroot>
<Tabelle11>
<Title_1>2009-10-12T00:00:00.000</Title_1>
<Title_2>40098</Title_2>
</Tabelle11>
<Tabelle11>
<Title_1>2000-01-01T00:00:00.000</Title_1>
<Title_2>36526</Title_2>
</Tabelle11>
</dataroot>
When I check the XML-Schema (e.g. with "Debug.Print ActiveWorkbook.XmlMaps(1).Schemas(1).XML") I see the difference (only snippet, not the complete file):
<xsd:element name="Title_1" minOccurs="0" od:jetType="datetime" od:sqlSType="datetime" type="xsd:dateTime">
<xsd:annotation>
<xsd:appinfo>
<od:fieldProperty name="Format" type="10" value="dd.mm.yyyy"/>
</xsd:appinfo>
</xsd:annotation>
</xsd:element>
<xsd:element name="Title_2" minOccurs="0" od:jetType="text" od:sqlSType="nvarchar">
<xsd:annotation>
<xsd:appinfo>
<od:fieldProperty name="Format" type="10" value="@"/>
</xsd:appinfo>
</xsd:annotation>
<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:maxLength value="255"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
The first column is in the schema of format datetime, the second is varchar. Again, you might not even be aware, how Excel handles your XML-data, since this schema is automatically generated (if I recall correctly in Excel 2003 there was a message indicating that an schema had benn generated for you)
Now how to acomplish this -- especially in an existing file: I would try to extract the XML-Schema (see above), save it as something.xsd, adapt the schema according to your needs and import this new Schema. Bummer is you have to set the mapping again (or try this: Excel 2007 XML Source Maps - Refreshing Schemas). Often I let MS Access generate my XML-Schema, i.e. generate the required table, insert some values and exprt the table as XML (ticking the option to add the schema).
I hope this general outline gives you an idea on how to proceed.
Regards
Andreas