question: public Period plusYears ( long yearsToAdd ) { if ( yearsToAdd == 0 ) { return this ; } return create ( Math . toIntExact ( Math . addExact ( years , yearsToAdd ) ) , months , days ) ; }
target:  Returns a copy of this period with the specified years added. <p> This adds the amount to the years unit in a copy of this period. The months and days units are unaffected. For example, "1 year, 6 months and 3 days" plus 2 years returns "3 years, 6 months and 3 days". <p> This instance is immutable and unaffected by this method call.