gistfile1.txt

//31.01.2000

// startDateOfContract = parameter
//customerAgeWhenContractEnd = parameter
//return end date of the contract = 1 day before the start date at the moment that the customer has the age he picked.

const calculateEndDateOfContract = (customerBirthDate, startDateOfContract, customerAgeWhenContractEnd) => {
  customerBirthDate = splitDate(customerBirthDate);
  startDateOfContract = splitDate(startDateOfContract);
  let endContractDate = new Date(customerBirthDate.year + customerAgeWhenContractEnd, startDateOfContract.month - 1, startDateOfContract.day - 1);
  let endContractDay = endContractDate.getDate();
  let endContractMonth = endContractDate.getMonth() + 1;
  let endContractYear = endContractDate.getFullYear();

  if(customerBirthDate.month > endContractMonth){
    endContractYear++;
  }else if(endContractMonth === customerBirthDate.month && endContractDate.getDate() < customerBirthDate.day){
    endContractYear++;
  }

  if(endContractMonth < 10){
    endContractMonth = `0${endContractMonth}`
  }
  if(endContractDay < 10){
    endContractDay = `0${endContractDay}`
  }

  return `${endContractDay}.${endContractMonth}.${endContractYear}`
};

const splitDate = (dateStr) =>{
  const splitDate = dateStr.split(".")
  return {
    day: parseInt(splitDate[0], 10),
    month:parseInt(splitDate[1], 10),
    year: parseInt(splitDate[2], 10)
  }
}
          
 console.log(calculateEndDateOfContract("15.09.2000", "15.09.2018", 65));
 console.log(calculateEndDateOfContract("16.09.2000", "15.09.2018", 65));
 console.log(calculateEndDateOfContract("14.09.2000", "15.09.2018", 65));