def dateFormat = 'yyyy-MM-dd'

Date.metaClass.nextDay = {
   use(groovy.time.TimeCategory) {
      def nDay = delegate + 1.day
      nDay.format(dateFormat)
   }
}

def dateString = '2017-12-14'
def date = Date.parse(dateFormat, dateString)
println date.nextDay()

def date2 = new Date()
println date2.nextDay()