KEHRSATZ, Switzerland — Prime Minister Stephen Harper signed a treaty here Friday that he says gives Canada an important new tool to track tax cheats who hide their money in Swiss accounts. But a new “double taxation” treaty Harper signed with Swiss president Doris Leuthard will only affect future taxation years, Canadian officials said, and won’t give Revenue Canada any new ways to track down a reported 1,800 Canadians who are hiding millions from Canada’s taxman… In the House of Commons, though, the NDP said Harper needs to do more to track rich tax cheats. “This government is letting wealthy tax evaders off the hook,” NDP MP Libby Davies told the Commons. “It is bragging about a tax treaty with Switzerland that will do little to recover billions of tax dollars hidden overseas by tax chiefs and corporations. What little information we do have on Canadian tax evaders is the result of the efforts of other countries, France and the United States.”