Calculation Methods & Accuracy Standards

function calculateChronologicalAge(birthDate: Date, assessmentDate: Date): AgeResult {
  // Normalize dates to UTC to avoid timezone issues
  const birth = new Date(birthDate.getFullYear(), birthDate.getMonth(), birthDate.getDate());
  const assessment = new Date(assessmentDate.getFullYear(), assessmentDate.getMonth(), assessmentDate.getDate());
  
  // Calculate years
  let years = assessment.getFullYear() - birth.getFullYear();
  
  // Calculate months
  let months = assessment.getMonth() - birth.getMonth();
  
  // Calculate days
  let days = assessment.getDate() - birth.getDate();
  
  // Handle negative days (assessment day < birth day)
  if (days < 0) {
    months--;
    const lastMonth = new Date(assessment.getFullYear(), assessment.getMonth(), 0);
    days += lastMonth.getDate();
  }
  
  // Handle negative months (assessment month < birth month)
  if (months < 0) {
    years--;
    months += 12;
  }
  
  return { years, months, days, isValid: true };
}

function validateAgeCalculation(birthDate: Date, assessmentDate: Date): boolean {
  // Method 1: Date arithmetic (primary)
  const result1 = calculateChronologicalAge(birthDate, assessmentDate);
  
  // Method 2: Millisecond difference validation
  const birthTime = birthDate.getTime();
  const assessmentTime = assessmentDate.getTime();
  const diffMs = assessmentTime - birthTime;
  const diffDays = Math.floor(diffMs / (1000 * 60 * 60 * 24));
  
  // Convert back to years/months/days for comparison
  const totalDays = result1.years * 365.25 + result1.months * 30.44 + result1.days;
  
  // Allow small variance due to calendar irregularities
  const variance = Math.abs(totalDays - diffDays);
  
  return variance < 2; // Less than 2 days variance acceptable
}