I cannot recall his exact title, but the Prince of Monaco just passed away. The news said something about him being the longest serving monarch, but since when and/or for what? Did they mean in Monaco’s history? Europe’s history?
Maybe some Striaght Doper can Dope me Straight… 
I read that it was in European history - Europe not including the U.K., since Victoria reigned for at least 60 years and Rainer was only 56.
He was at the time of his death the current European monarch who had reigned the longest. There are others in history with much longer reigns, European and otherwise, and IIRC there’s one non-European monarch (Thailand?) who’s been on his throne longer. But what they said and meant was that of all the European monarchs, he’d reigned the longest as of the time of his death. (Elizabeth II is now the person in that role.)