Calculating difference between two years in excel with if else condition

I want to calculate difference of second date (year) and first date(year). I cannot write formula in excel.

if(quitDate is not empty)
{
  quitDate - startDate
}
else 
{
  presentDate - startDate
}

I know only these two formulas =YEAR(NOW)-YEAR(A1) and = YEAR(B1)-YEAR(A1)