You could query the event log in question:
var sourceName = "MySource";
var el = new EventLog("Application");
var latestEntryTime = (from entry in el.Entries.Cast()
where entry.Source == sourceName
&& // put other where clauses here...
orderby entry.TimeWritten descending
select entry).First();
However, be warned that this approach is slow, since the Entries collection tends to be quite big.