the difference between the chosen course and the one in a file on a specific date
This could be done *without* actually parsing the XML file [think how various text comparison programs work]...I wonder if such an approach would be accepted.

As stated previously, writing an XML Parser is a HUGE undertaking. Writing a parser for a fixed file format (which might happen to be XML) is a different task...

You are 100% correct in "I think I first have to figure out what exactly should this program do."