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@params [year: integer]
@params [month: integer, nil]
@params [day: integer, nil]
@return[Time] a UTC time (Uses Time instead of Date so that it can be saved as a UTC object -
  See http://www.ruby-doc.org/core-2.0.0/Time.html)
  Returns nomenclature_date based on computation of the values of :year, :month, :day.
   if :year is empty, return nil
   if :month is empty, returns 12/31/:year
   if :day is empty, returns the last day of the month

Use self.month_index to convert months prior to handling them here
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