Using preceding-sibling::node()

Hi,

I'm fairly new to XML and I'm trying to modify my query so that it fetches the preceding sibling to
the originally requested node if the originally requested node does not exist.
For example:
if my original query was something like
 child::data[attribute::id = 'archive'][@id='archive']/child::timestamp[attribute::time = '1099119599']

and if the node with that particular timestamp is not present in the XML document I'd like to modify my query
in such a way that it fetches the node that has a timestamp < 1099119599.

I understand if I change my query to
child::data[attribute::id = 'archive'][@id='archive']/child::timestamp[attribute::time < '1099119599']
then all the nodes with timestamp < 1099119599 get returned

Any help would be sincerely appreciated. Thanks

-Dilip