Question: When did the conference the UNLV Runnin' Rebels are a member of begin operations?

Context paragraphs:
2010–11 UNLV Runnin' Rebels basketball team: The 2010–11 UNLV Runnin' Rebels basketball team represented the University of Nevada, Las Vegas.  The team was coached by Lon Kruger, returning for his seventh year with the Runnin' Rebels.  They played their home games at the Thomas & Mack Center on UNLV's main campus in Paradise, Nevada and are a member of the Mountain West Conference.  They finished the season 24–9, 11–5 in Mountain West play and lost in the semifinals of the 2011 Mountain West Conference Men's Basketball Tournament to San Diego State.  They received an at-large bid in the 2011 NCAA Men's Division I Basketball Tournament where they lost in the second round to Illinois.

Mountain West Conference: The Mountain West Conference (MW) is one of the collegiate athletic conferences affiliated with the National Collegiate Athletic Association (NCAA) Division I Football Bowl Subdivision (FBS) (formerly I-A).  The MW officially began operations in July 1999.  Geographically, the MW covers a broad expanse of the Western United States, with member schools located in California, Colorado, Hawaii, Idaho, Nevada, New Mexico, Utah, and Wyoming.  Craig Thompson has served as Commissioner of the MW since its founding in 1999.

2017–18 UNLV Runnin' Rebels basketball team: The 2017–18 UNLV Runnin' Rebels basketball team will represent the University of Nevada, Las Vegas during the 2017–18 NCAA Division I men's basketball season.  The Runnin' Rebels will be led by second-year head coach Marvin Menzies.  They will play their home games at the Thomas & Mack Center in Paradise, Nevada as members of the Mountain West Conference.

2012–13 UNLV Runnin' Rebels basketball team: The 2012–13 UNLV Runnin' Rebels men's basketball team represented the University of Nevada, Las Vegas during the 2012–13 NCAA Division I men's basketball season.  The team was coached by Dave Rice, in his second year with the Runnin' Rebels.  They played their home games at the Thomas & Mack Center on UNLV's main campus in Las Vegas, Nevada and were a member of the Mountain West Conference.  They finished with a record of 25–10 overall, 10–6 in Mountain West play to finish in third place.  They advanced to the championship game of the Mountain West Tournament where they lost to New Mexico.  They receive an at-large bid in the 2013 NCAA Tournament where they lost in the second round to California.

2011–12 UNLV Runnin' Rebels basketball team: The 2011–12 UNLV Runnin' Rebels men's basketball team represented the University of Nevada, Las Vegas.  The team was coached by Dave Rice, in his first year with the Runnin' Rebels.  They played their home games at the Thomas & Mack Center on UNLV's main campus in Las Vegas, Nevada and are a member of the Mountain West Conference.  UNLV's season ended with 26–9 overall, and 9–5 in MWC Play, placing third.  They lost in the semifinals of the Mountain West Basketball Tournament by New Mexico.  They received an at-large bid to the 2012 NCAA Tournament where they lost in the second round to Colorado.

2009–10 UNLV Runnin' Rebels basketball team: The 2009–10 UNLV Runnin' Rebels basketball team represented the University of Nevada, Las Vegas.  The team was coached by Lon Kruger, returning for his sixth year with the Runnin' Rebels.  They played their home games at the Thomas & Mack Center on UNLV's main campus in Paradise, Nevada and are a member of the Mountain West Conference.  The Runnin' Rebels finished the season 25–9, 11–5 in MWC play.  They advanced to the championship game of the 2010 Mountain West Conference Men's Basketball Tournament before losing to San Diego State.  They received an at–large bid to the 2010 NCAA Men's Division I Basketball Tournament, earning an 8 seed in the Midwest Region, where they lost to 9 seed Northern Iowa in the first round.

2013–14 UNLV Runnin' Rebels basketball team: The 2013–14 UNLV Runnin' Rebels basketball team represented the University of Nevada, Las Vegas during the 2013–14 NCAA Division I men's basketball season.  The team was coached by Dave Rice, in his third year with the Runnin' Rebels.  They played their home games at the Thomas & Mack Center on UNLV's main campus in Paradise, Nevada and were a member of the Mountain West Conference.  They finished the season 20–13, 10–8 in Mountain West play to finish in a tie for third place.  They advanced to the semifinals of the Mountain West Conference Tournament to San Diego State.  They did not play in a postseason tournament for the first time since 2009.

2014–15 UNLV Runnin' Rebels basketball team: The 2014–15 UNLV Runnin' Rebels basketball team represented the University of Nevada, Las Vegas during the 2014–15 NCAA Division I men's basketball season.  The Runnin' Rebels were led by fourth year head coach Dave Rice.  They played their home games at the Thomas & Mack Center in Paradise, Nevada as members of the Mountain West Conference.  They finished the season 18–15, 8–10 in Mountain West play to finish in seventh place.  They advanced to the quarterfinals of the Mountain West Tournament where they lost to San Diego State.

2016–17 UNLV Runnin' Rebels basketball team: The 2016–17 UNLV Runnin' Rebels basketball team represented the University of Nevada, Las Vegas during the 2016–17 NCAA Division I men's basketball season.  The Runnin' Rebels were led by first-year head coach Marvin Menzies.  They played their home games at the Thomas & Mack Center in Paradise, Nevada as members of the Mountain West Conference.  They finished the season 11–21, 4–14 in Mountain West play to finish in a tie for tenth place.  They lost in the first round of the Mountain West Tournament to San Diego State.

1989–90 UNLV Runnin' Rebels basketball team: The 1989–90 UNLV Runnin' Rebels basketball team represented the University of Nevada Las Vegas in NCAA Division I men's competition in the 1989–90 season, and won the NCAA title under head coach Jerry Tarkanian.  The team played its home games in the Thomas & Mack Center, and was a member of the Big West Conference; it would join the Western Athletic Conference in 1996 and become a charter member of its current conference, the Mountain West Conference, in 1999.  As of the present, they are the last team from any of the non-big six conferences (ACC, Big East, Big Ten, Big 12, Pac-10, and SEC) to win the national championship.

Supporting paragraphs: 1989–90 UNLV Runnin' Rebels basketball team Mountain West Conference
Answer: July 1999