Date Difference Calculator

Method 1: Subtract timestamps in milliseconds and divide by 86,400,000 (ms per day). JavaScript: Math.floor((new Date(end) - new Date(start)) / 86400000). Method 2: Use the difference in UTC midnight timestamps to avoid daylight saving time issues. Tip: always use Date.UTC() or parse ISO dates to avoid timezone offset bugs. In Python: (date2 - date1).days. The result is the inclusive number of days from start to end.

Business days exclude weekends (Saturday, Sunday) and optionally public holidays. Calculating business days between two dates: iterate each day, skip weekends. Formula without iteration: business days = total days - (complete weeks × 2) - partial week weekends. For holidays, maintain a list of dates to skip. International note: some countries work on Saturday, and holiday calendars differ. Monday is typically the first business day in most countries.

JavaScript: new Date(date.getTime() + days × 86400000) — avoids DST issues by using milliseconds. Or: date.setDate(date.getDate() + days) — modifies in place. Python: date + timedelta(days=N). SQL: DATE_ADD(date, INTERVAL N DAY) (MySQL) or date + N (PostgreSQL). Adding months is trickier (month lengths differ): date.setMonth(date.getMonth() + N) in JavaScript handles this correctly, rolling over to the next month if needed (March 31 + 1 month = April 30, not April 31).

Common pitfalls: Timezone issues — new Date("2024-01-15") parsed as UTC midnight, different from new Date("2024-01-15T00:00:00") (local time). DST (Daylight Saving Time) — days can be 23 or 25 hours, so multiplying hours by days gives wrong results. Month-end arithmetic — adding 1 month to Jan 31 could give Feb 28 or March 2. Leap year handling — Feb 29 doesn't exist in non-leap years. Best practice: use ISO 8601 UTC timestamps and tested date libraries (date-fns, Luxon) for critical calculations.

ISO 8601 is the international standard for date and time representation: dates as YYYY-MM-DD (2024-01-15), times as HH:mm:ss, combined: 2024-01-15T10:30:00Z (UTC). The format is unambiguous — unlike 01/15/24 which could be Jan 15 or 15 January depending on locale. ISO 8601 sorts correctly alphabetically. Duration format: P1Y2M3DT4H5M6S (1 year 2 months 3 days 4 hours 5 minutes 6 seconds). Interval: 2024-01-01/2024-12-31.