Off the top of my head, how about:
1. Get a list of content items sorted by publication_date DESC:
GetContent(int from, int max, string sortExp)
2. Then take that top item, and repeat the query, this time adding the publication_date of the initial item to your filter array.
In any case, you have to query the provider to find out what the latest date is. There might be a more efficient way, but simply getting one record, sorted by publication_date seems simplest to me.