Date Calculator - 無料オンライン電卓

A date calculator is one of those utilities you don't realise you need until you do - figuring out a project deadline 45 working days from today, the due date for a 9-month gestation pregnancy from the LMP date, when a 21-day credit period ends, when a 5-year fixed deposit matures, the expiry date of a 6-month visa, or what day of the week a child's birthday falls on next year. Indian professionals deal with date math constantly: GST return deadlines (20th of next month), income tax filing dates (31 July, 30 September), TDS deposit dates (7th of next month), and PF/ESI compliance dates. This calculator handles all standard date arithmetic with built-in support for business-days-only mode.

Two operations: add or subtract a duration (in days, weeks, months, years) from a starting date. The result shows the new date along with the weekday name, which is often what you actually need ('is this a Saturday?'). Business-days mode is critical for project planning where Saturdays and Sundays don't count toward delivery timelines. The calculator runs entirely client-side using JavaScript's Date API which correctly handles leap years, varying month lengths (28/29/30/31 days), and edge cases like adding 1 month to 31 January (rolls back to 28/29 February).

Day-based arithmetic: simple counter math. Adding 30 days advances exactly 30 calendar days regardless of month boundaries. Weeks: multiply by 7 and add days. The Date API handles month-end carry automatically (1 Jan + 60 days = 2 March in a non-leap year).

Month/year arithmetic: tricky because months have varying lengths. Convention: preserve the day of month where possible; if not, clamp to the last day of the resulting month. So 31 January + 1 month = 28 February (or 29 in a leap year) rather than overflowing to March 3. 31 March + 1 month = 30 April. This is the standard 'calendar-aware' behaviour used in legal documents, lease agreements and financial maturity dates.

Business days mode: increment skips Saturdays and Sundays. If today is Friday and you add 5 business days, the result is Friday of the following week (5 weekdays later), not 5 calendar days later. Note: this calculator doesn't auto-subtract public holidays (which vary by state/country). For India, you may need to add a few days margin for major festivals (Diwali week, holi, regional new year). For strict legal deadlines, always check the official holiday calendar for the relevant jurisdiction.