Data Manipulation in XML files

I have 5 XML files with about 1,200 calendar event entries per file. For each entry, I need a node element value changed based on the content of another node. Specifically, if the <local-end-date-time> node does not contain an end time then the <hide-end> node needs to be changed to Y. For example, <local-end-date-time>8/10/2015 </local-end-date-time> would result in <hide-end>Y</hide-end> for that event node. But, <local-end-date-time>8/12/2015 7:30 PM</local-end-date-time> would result in <hide-end>N</hide-end> (the default). There are 9 element nodes between the node being evaluated and the one being changed. For the right person, this is a simple script to write and run on my files. It doesn't matter if the work is done manually, but it needs to be error free.