Question: How long did Greg Gianforte's predecessor serve as U.S. representative for Montana's at-large congressional district?

Context paragraphs:
Ryan Zinke: Ryan Keith Zinke (born November 1, 1961) is an American politician and businessman who is currently the 52nd United States Secretary of the Interior, serving in the Trump administration.  Zinke served as the U.S. representative for Montana's at-large congressional district from 2015 until 2017.  From 2009 to 2013, he served as a member of the Montana Senate, representing the 2nd district.

Utah Territory's at-large congressional district: Utah Territory's At-large congressional district is an obsolete congressional district that encompassed the area of the Utah Territory.  After Utah's admission to the Union as the 45th state by act of Congress on January 4, 1896, this district was dissolved and replaced by Utah's At-large congressional district.

Chris Nelson (politician): Chris Nelson is the former Secretary of State of South Dakota.  A Republican, he was a candidate to become U.S. Representative from South Dakota's At-large congressional district in 2010 but lost to Kristi Noem in the Republican primary.

Kim Gillan: Kim J. Gillan is a former Democratic Party member of the Montana Senate.  She represented District 24 from 2004 to 2012.  She was unable to run for reelection in 2012 due to Montana's term limits.  Earlier she was a member of the Montana House of Representatives from 1996 through 2004.  On June 21, 2011, she announced that she would be a candidate for the U.S. House of Representatives for the open seat in Montana's At-large congressional district that was available in the 2012 election due to incumbent Denny Rehberg's decision to run against U.S. Senator Jon Tester.  Gillan was defeated by Republican businessman Steve Daines in the November 2012 general election.

Iowa Territory's at-large congressional district: Iowa Territory's At-large congressional district is an obsolete congressional district that encompassed the area of the Iowa Territory, which was split off from the Wisconsin Territory in 1838.  After Iowa's admission to the Union as the 29th state by act of Congress on December 28, 1846, this district was dissolved and replaced by Iowa's At-large congressional district.

Oregon's at-large congressional district: Oregon's At-large congressional district is a former United States Congressional District.  During its existence, Oregon voters elected a Representative of the United States House of Representatives At-large from the entire state.

Tom Stout: Tom Stout (May 20, 1879 – December 26, 1965) was a U.S. Representative from Montana, who represented Montana's at-large congressional district from March 4, 1913, to March 3, 1917.

Greg Gianforte: Gregory Richard Gianforte (born April 17, 1961) is an American businessman, engineer, and politician who is the U.S. Representative for Montana's at-large congressional district.  On May 25, 2017, he won the special election following Rep. Ryan Zinke's resignation to become Secretary of the Interior.  Gianforte and his wife founded RightNow Technologies, a customer relationship management software company.

Alabama's at-large congressional district: Alabama's at-large congressional district was a congressional district for the United States House of Representatives in Alabama active at various times from 1819 to 1965.  Alabama became a state in 1819, and its single representative to the 16th and 17th Congresses was elected at-large.  For the 27th Congress, all five of Alabama's representatives were elected at-large, before the state gained a representative from the 1840 census.  In the 43rd to 44th Congresses, the seventh and eighth representatives gained in the 1870 census were elected at-large.  For the 63rd and 64th Congresses, Alabama elected the tenth of its apportioned representatives, gained in the 1910 census, at-large from the entire state.  For the 88th Congress, after the state lost one representative in the 1960 census, Alabama once again elected all of their representatives at-large.

Montana's at-large congressional district: Montana is represented in the US House of Representatives by one at-large congressional district, among the 435 in the U.S. Congress.  The district is the largest U.S. congressional district by population, with just over 1 million constituents.  It is also the second-largest by land area, after Alaska's at-large congressional district.

Supporting paragraphs: Greg Gianforte Ryan Zinke
Answer: 2015 until 2017