Date / Age Calculator bundles four related date calculations: finding your exact age in years, months, and days; finding the calendar-aware difference between two dates; adding or subtracting days, weeks, months, or years from a date; and counting weekdays between two dates for scheduling or work-day estimates.

All four modes use native JavaScript date arithmetic and account for real calendar irregularities — varying month lengths and leap years — rather than approximating with a fixed number of days per month or year.

The age calculation is a good example of why calendar-aware math matters: naively dividing the number of days since a birth date by 365.25 produces an age that's subtly wrong for most people, most of the time, because it ignores where your birthday actually falls relative to leap years. This tool instead counts whole elapsed years, then whole elapsed months within the current year, then whole elapsed days within the current month — the same way you'd count it by hand on a calendar — so the years/months/days breakdown always matches what you'd get counting manually.

The add/subtract mode follows JavaScript's native calendar-rollover behavior: adding a month to a date near month-end that doesn't exist in the target month (like January 31st, where February has no 31st day) rolls forward into the following month rather than silently clamping to an earlier date — January 31 plus one month lands on March 3 in a non-leap year, not February 28. It's a deliberate choice to surface this "spillover" clearly rather than hide it behind a clamped date that looks tidier but quietly loses a few days. Business-day counting is deliberately scoped to Monday–Friday only — it doesn't attempt to model regional public holidays, since those vary by country and even by year, and silently guessing wrong would be worse than not guessing at all.