question: public Period plusDays ( long daysToAdd ) { if ( daysToAdd == 0 ) { return this ; } return create ( years , months , Math . toIntExact ( Math . addExact ( days , daysToAdd ) ) ) ; }
target:  Returns a copy of this period with the specified days added. <p> This adds the amount to the days unit in a copy of this period. The years and months units are unaffected. For example, "1 year, 6 months and 3 days" plus 2 days returns "1 year, 6 months and 5 days". <p> This instance is immutable and unaffected by this method call.