Date Calculator

Find how many days are between two dates, or calculate what date falls a specific number of days from today.

Date Difference

Add / Subtract Days

Date Calculations: Essential for Planning, Legal, and Medical Use

Knowing the exact number of days between dates — or finding a specific future or past date — is critically important in dozens of practical contexts. The irregular nature of the Gregorian calendar (months of 28, 29, 30, or 31 days; leap years every 4 years with exceptions) makes manual date arithmetic error-prone. The UltraTools Date Calculator handles all calendar complexity automatically.

Common Use Cases

  • Project deadlines: "We have 45 business days to deliver. What is the deadline?"
  • Contract terms: "A 90-day payment period from invoice date of March 15 — when is payment due?"
  • Medical schedules: "The next follow-up appointment is in 21 days — what specific date is that?"
  • Travel planning: "I can stay in the Schengen Area for up to 90 days. When must I leave?"
  • Pregnancy: "My due date is 280 days from my last menstrual period — what is the date?"
  • Age milestones: "Exactly how many days until I turn 30?"
  • Legal deadlines: Court filing deadlines, appeal windows, statute of limitations calculations

How the Calculator Works

For date differences: the calculator converts both dates to milliseconds since January 1, 1970 (the Unix epoch), computes the absolute difference, and divides by 86,400,000 (the number of milliseconds per day) to get the total day count. Weeks, months, and years are derived from this total.

For date addition: the calculator creates a Date object from the start date, adds the specified days to the day component (which automatically handles month and year rollovers), and formats the resulting date for display.

Tips for Accurate Date Calculations

💡 Tip: For legal deadlines, always verify whether the counting method is "inclusive" (count the first day) or "exclusive" (do not count the first day). This single decision can shift a deadline by one day and has legal consequences.
💡 Tip: When calculating periods spanning February, always account for whether Feb 28 or Feb 29 applies (leap year check). Our calculator handles this automatically based on the actual year.

Frequently Asked Questions

Yes, completely automatically. JavaScript's Date object accounts for all leap years in the Gregorian calendar, including the exception cases (years divisible by 100 are not leap years unless also divisible by 400). For example, 2000 was a leap year, but 1900 was not — our calculator handles both correctly.
The current tool calculates calendar days only. For a rough approximation of business days, multiply the calendar day count by 5/7 (approximately 0.714) and subtract public holidays that fall in the period. A more precise business day calculator is planned for a future update.
The calculator handles both positive (future) and negative (past) values correctly. Entering -100 will return the date 100 days before the start date, even if that crosses year boundaries or goes back multiple months. The result will always be a valid calendar date.