How do I calculate the number of days between two dates?

Use the Date Difference tab. Enter a start date and end date and the calculator instantly returns the exact number of days, weeks, months, and years between them. It accounts for leap years and varying month lengths automatically.

How do I add days to a date?

Use the Add or Subtract Days tab. Enter a start date, enter the number of days to add (or a negative number to subtract), and the calculator returns the resulting date along with the day of the week.

How many days until Christmas 2026?

Use the Days Until Event tab and select Christmas, or enter December 25, 2026 as the target date. The calculator shows the exact number of days, weeks, and months remaining from today.

How do I calculate business days between two dates?

Use the Business Days tab. Enter a start and end date and the calculator counts only Monday through Friday, excluding weekends. It does not exclude public holidays by default, but you can manually adjust the result for known holidays.

What is the day number of the year for any date?

Use the Day of Year tab. Enter any date and the calculator returns what day number it is out of 365 (or 366 in a leap year), plus how many days remain in the year.

How many days until New Year 2027?

Use the Days Until Event tab and select New Year, or enter January 1, 2027 as the target date. The result shows the exact days, weeks, and months remaining from today's date.

How do I find what date was 90 days ago?

Use the Add or Subtract Days tab. Enter today as the start date and type -90 (negative ninety) in the days field. The calculator returns the exact date 90 days before today.

What is a date difference calculator used for?

Date difference calculators are used for project deadline tracking, legal contract term calculations, medical treatment intervals, pregnancy due date estimation, lease and rental period calculations, court filing deadlines, and calculating time between events like anniversaries or employment start dates.

How many weeks between two dates?

The Date Difference tab returns the result in both days and complete weeks automatically. Divide the total days by 7 to get weeks, or use the calculator which shows both units in the breakdown grid.

What does adding or subtracting dates mean?

Date addition means finding the date that results from adding a number of days, weeks, or months to a starting date. For example, adding 45 days to January 15 gives March 1. Date subtraction finds the date that comes before a start date by a certain number of days. Both operations account for month lengths and leap years.

Can I calculate working days only, excluding weekends?

Yes. Use the Business Days tab which counts only Monday through Friday between your two dates. This is useful for contract notice periods, payment terms, shipping estimates, and project timelines that operate on business schedules.

How many days until my deadline?

Use the Days Until Event tab. Enter your deadline date as the target and the calculator instantly shows how many days, business days, weeks, and months remain from today until that deadline.

Date Calculator Date Difference, Add Days & Countdown

📅 Select Calculation Mode

Start Date

Earlier date Please enter a valid start date.

End Date

Later date Please enter a valid end date.

Start Date

Date to add or subtract from Please enter a valid date.

Days to Add (or Subtract)

days

Use negative number to subtract (e.g. -30) Enter a number of days (negative to subtract).

Quick Select Event Or enter any date below

Target Date

The date you are counting down to Please enter a future date.

Start Date

First day of the period Please enter a valid start date.

End Date

Last day of the period Please enter a valid end date.

Date

Find the day number in the year Please enter a valid date.

Date Difference

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⚠️ Disclaimer: This calculator uses the Gregorian calendar. Business days calculation excludes weekends but does not automatically account for public holidays. Results are for informational purposes only.

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Sources & Methodology

✓All date arithmetic follows the Gregorian calendar standard. Business day calculations verified against U.S. federal working day conventions. ISO 8601 date format used for all internal computation.

🔭

U.S. Naval Observatory — Gregorian Calendar Definition

Authoritative source for Gregorian calendar rules including leap year formula: divisible by 4, except century years unless divisible by 400. All date arithmetic in this calculator follows this standard.

🏢

U.S. Office of Personnel Management — Federal Holidays

Reference for standard U.S. federal holiday schedule. The business days calculator excludes Saturdays and Sundays. Public holidays are not automatically excluded but can be found on this OPM resource for manual adjustment.

📋

ISO 8601 — International Date and Time Standard

All internal calculations use ISO 8601 YYYY-MM-DD format ensuring unambiguous, internationally consistent date arithmetic across all five modes.

Methodology:
Date Difference = Count all calendar days from Start Date to End Date (inclusive of leap days) Add Days: Result Date = Start Date + N calendar days (handles month/year rollovers and leap years) Business Days = Total Days − (Total Weeks × 2) − Partial Weekend Days Day of Year = Sum of days in each completed month + current day number Leap year rule: year divisible by 4, except century years unless divisible by 400. February 29 is counted only in leap years.

Last reviewed: April 2026

Complete Guide to Date Calculation — Every Use Case Covered

Date calculation is one of the most frequently needed everyday math tasks. Whether you are tracking a project deadline, counting down to a holiday, calculating a contract term, or figuring out how many business days are left in a notice period — this date calculator handles every scenario. Here is a complete breakdown of every calculation mode and when to use it.

Date Difference — How Many Days Between Two Dates

The date difference calculator counts every calendar day between a start and end date, including leap days. It returns the result in days, complete weeks, complete months, and full years. This is the most commonly used date calculation for project planning, legal contract terms, medical treatment intervals, rental and lease period tracking, and verifying time elapsed between events.

Date Difference Formula

Total Days: Count every calendar day from the start date to the end date. Each year contributes 365 days (or 366 in a leap year).
Complete Weeks: Total Days ÷ 7, rounded down
Complete Months: Count month boundaries crossed from start to end
Example: January 15, 2025 to July 4, 2026 = 535 days = 76 complete weeks = 17 months and 19 days

Add or Subtract Days from a Date

Date addition and subtraction finds a resulting date by adding or removing a number of days from a starting date. Enter a negative number to go backward in time. Common uses include finding a date 30, 45, 60, or 90 days from today (contract terms, payment deadlines, insurance notice periods, subscription renewals), and calculating expiration dates, due dates, and follow-up appointment dates. The calculator correctly handles month-end rollovers and leap year boundaries.

Common Use Case	Days to Add	Example Result (from Apr 14, 2026)
Net 30 payment term	+30	May 14, 2026
Net 45 invoice due	+45	May 29, 2026
60-day notice period	+60	June 13, 2026
90-day probationary period	+90	July 13, 2026
6-month lease term	+183	October 14, 2026
1 year from today	+365	April 14, 2027
30 days ago	−30	March 15, 2026
90 days ago	−90	January 14, 2026

Days Until an Event — Countdown Calculator

The countdown mode calculates the exact number of days between today and any future date. It is commonly used for holiday countdowns (days until Christmas, New Year, Thanksgiving, Halloween, Valentine's Day), wedding planning, event organization, tax filing deadlines, visa expiration dates, warranty end dates, and retirement milestone tracking. The calculator also shows how many complete weeks and months remain alongside the day count.

💡 2026 Holiday Countdowns (from April 14, 2026): Days until July 4th Independence Day: 81 days • Days until Halloween: 200 days • Days until Thanksgiving (Nov 26): 226 days • Days until Christmas: 255 days • Days until New Year 2027: 261 days. Use the Days Until tab above to calculate from any date.

Business Days Calculator — Working Days Only

The business days calculator counts only Monday through Friday between two dates, excluding all Saturdays and Sundays. This is essential for legal notice periods (many contracts specify business days, not calendar days), employment notice requirements, court filing deadlines, government application processing times, shipping and delivery estimates, and financial settlement periods. Note that public holidays are not automatically excluded — manually subtract known holidays in your jurisdiction from the result.

Day of the Year — What Number Day Is Today?

The day of the year calculator tells you what ordinal day number any date falls on in the calendar year. January 1 is day 1, December 31 is day 365 (or 366 in a leap year). This is used in scientific and agricultural contexts, financial reporting, production scheduling, and any system that uses Julian date formats. The calculator also shows how many days remain in the year after the entered date.

Date Calculation in Legal and Professional Contexts

Many legal and professional time limits are expressed in days rather than calendar months. Knowing the difference matters significantly. A 30-calendar-day notice period starting April 14 ends May 14. A 30-business-day period (excluding weekends) starting April 14 ends approximately June 25. For legal, employment, or financial matters, always confirm whether the applicable rule counts calendar days or business days. When in doubt, use both calculations and consult a professional.

Real estate closings: Typically use calendar days for inspection and contingency periods
Employment contracts: Notice periods may specify business days or working days
Court deadlines: Federal courts often follow specific day-counting rules where the triggering day is excluded
Insurance claims: Filing windows are usually calendar days
Payment terms: Net 30, Net 45, Net 60 typically mean calendar days in most business agreements

Frequently Asked Questions

Use the Date Difference tab. Enter a start date and end date and the calculator returns the exact number of days, complete weeks, and months between them. It handles leap years and all month lengths automatically.

Use the Add or Subtract Days tab. Enter a start date and the number of days to add. For subtraction, enter a negative number (e.g. -30 for 30 days before). The calculator returns the resulting date and what day of the week it falls on.

Use the Days Until Event tab and select Christmas from the quick-select menu, or enter December 25, 2026 as the target date. The result shows exact days, weeks, and months remaining from today.

Use the Business Days tab. Enter a start and end date and the calculator counts only Monday through Friday, excluding all weekends. Note that public holidays are not automatically excluded. Subtract any known holidays from the result for an exact business day count.

Use the Add or Subtract Days tab, enter today as the start date, and type 30 in the days field. The calculator returns the exact calendar date 30 days from today with the day of the week. You can also use 45, 60, 90, or any number of days.

Use the Add or Subtract Days tab with today as the start date and 90 as the number of days. The calculator returns the exact date 90 calendar days from today. This is commonly used for probationary period end dates and 90-day payment terms.

Use the Days Until Event tab and select New Year 2027 from the quick-select menu, or manually enter January 1, 2027. The calculator shows exact days, weeks, and months remaining from today's date.

Calendar days count every day including weekends and holidays. Business days count only weekdays (Monday through Friday), typically excluding public holidays. A 30-calendar-day period and a 30-business-day period can differ by 8 to 12 days depending on how many weekends fall within the range.

In non-leap years, day 100 is April 10. In leap years, day 100 is April 9. Use the Day of Year tab to look up any date, or enter a date to find what ordinal number it is in the year.

Use the Add or Subtract Days tab. Enter the contract start date and add the number of days in the contract term (e.g. 365 for one year, 183 for six months, 90 for three months). The result is the exact contract end date including leap year adjustments.

Yes, fully accurate. The calculator applies the Gregorian leap year rule: divisible by 4, except century years unless divisible by 400. February 29 is correctly counted in leap years (2024, 2028, 2032, etc.) and not counted in other years.

Yes. The Date Difference tab returns the total in both days and complete weeks. For example, 70 days is 10 complete weeks. Partial weeks are shown as remaining days beyond the complete week count.

Use the Days Until Event tab and enter your deadline as the target date. The calculator shows the exact number of calendar days, business days (approximate), weeks, and months remaining from today until your deadline.

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