Date Calculator

Subtract the earlier date from the later date in milliseconds and divide by 86,400,000 (ms per day). In JavaScript: Math.round((new Date(end) - new Date(start)) / 86400000). Whether you include both start and end dates depends on convention — for "how many days does an event span", include both; for "how many days until", exclude the start date.

Business days exclude Saturdays and Sundays. To count them, iterate day by day between the two dates and count days where getDay() is not 0 (Sunday) or 6 (Saturday). For more precise calculations that also exclude public holidays, you need a country-specific holiday calendar, which this tool does not include.

Adding months requires care with month-end dates. Adding 1 month to January 31 should give February 28/29, not March 2/3. Most date libraries handle this by clamping to the last valid day of the resulting month. In JavaScript: set the month with setMonth(), then check if the day changed — if so, set back to the last day of the previous month.

A Unix timestamp is the number of seconds (or milliseconds in JavaScript) since January 1, 1970 00:00:00 UTC. It provides a timezone-independent reference point. new Date().getTime() returns milliseconds since epoch. Unix timestamps are always in UTC — they have no timezone. To display them, convert to a Date object and format in the desired timezone.

Age = current year - birth year. But you must subtract 1 if the birthday hasn't occurred yet this year (month/day comparison). A reliable method: start with (currentYear - birthYear), then subtract 1 if today is before the birthday month/day combination. This handles leap year birthdays correctly in most cases.