Date Calculations Guide: Mastering Date Arithmetic and Duration Calculations

Month End Handling

Issue: Different month lengths affecting calculations

Solution: Implement end-of-month rules and proper month arithmetic

Leap Year Calculations

Issue: Extra day in leap years affecting durations

Solution: Use date libraries that handle leap years automatically

Working Days Calculation

Issue: Excluding weekends and holidays

Solution: Maintain holiday calendar and implement business day logic

Timezone Considerations

Issue: Calculations crossing timezone boundaries

Solution: Perform calculations in UTC and convert results to local time

Basic Duration Calculation

// Using date-fns library
import { differenceInDays, differenceInMonths, differenceInYears } from 'date-fns';

const startDate = new Date('2024-01-01');
const endDate = new Date('2024-12-31');

const days = differenceInDays(endDate, startDate);
const months = differenceInMonths(endDate, startDate);
const years = differenceInYears(endDate, startDate);

Working Days Calculation

// Calculate working days between dates
function getWorkingDays(startDate, endDate, holidays = []) {
  let count = 0;
  const currentDate = new Date(startDate);
  
  while (currentDate <= endDate) {
    // Check if it's a weekday (Monday-Friday)
    if (currentDate.getDay() !== 0 && currentDate.getDay() !== 6) {
      // Check if it's not a holiday
      const isHoliday = holidays.some(holiday => 
        holiday.getDate() === currentDate.getDate() &&
        holiday.getMonth() === currentDate.getMonth()
      );
      if (!isHoliday) {
        count++;
      }
    }
    currentDate.setDate(currentDate.getDate() + 1);
  }
  return count;
}