It is year here as it is a modifier here. I would say it this way: Monday, August 9, 2013 was the first day of the four year journey for a college diploma.
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Monday, August 9, 2013 was the first day of the four year journey for a college diploma.
Anonymous Monday, August 9, 2013 was the first day to the start of a four year journey towards a college diplomaMonday, August 9, 2013 marks the first day of my 4 year journey toward a college degree.