if (!aDate.isBefore(plan.summerStart) && !aDate.isAfter(plan.summerEnd)) {
  charge = quantity * plan.summerRate;
  }
else {
  charge = quantity * plan.regularRate + plan.regularServiceCharge;
}

• if then else
• 조건(if), then, else 가 길어지면 함수로 추출한다.

if (Summer(date)) {
    charge = summerCharge(quantity);
}
else {
	charge = winterCharge(quantity);
}

reference

https://www.refactoring.com/catalog/decomposeConditional.html