XML cross references and validation

Hi,
   Is there any way to parse XML with DOM for cross referencing of elements
Example:
<Date id ="one">
<year>1978</year>
<month>2</month>
<day>2</day>
</Date>

<Date id ="two">
<year>1978</year>
<month>2</month>
<day>2</day>
</Date>

<DateInterval id ="three">
<start>one</start>
<limit>two</limit>
</DateInterval>

Using DOm how cam I parse one and two elements like year etc???

Thanks
Bhaskar