Date Calculator
Add or subtract days, weeks, months, or years from any date.
Sunday
Sunday, February 8, 2026
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About This Calculator
The Date Calculator adds or subtracts any number of days, weeks, months, or years from any starting date, helping you answer questions like "what date is 90 days from today?" or "what was the date 6 months ago?" Whether you're calculating contract deadlines, planning project timelines, determining due dates, scheduling future events, or tracking historical milestones, this tool provides instant, accurate results. Our calculator properly handles the complexities of calendar math including varying month lengths (28, 29, 30, or 31 days), leap years, and end-of-month edge cases where adding a month might result in a different day of the month. For business applications, you can calculate in business days to exclude weekends, giving you realistic deadline projections. The date calculator is essential for legal professionals (statute of limitations, filing deadlines), project managers (milestone planning, sprint scheduling), HR departments (probation periods, anniversary dates), and anyone who needs to work with dates precisely. From simple "add 7 days" calculations to complex "add 18 months and 15 days" scenarios, this tool handles all date arithmetic with ease.
How to Use the Date Calculator
- 1Enter your starting date using the date picker or type in MM/DD/YYYY format.
- 2Select whether you want to add or subtract time from the starting date.
- 3Enter the number of days, weeks, months, or years to add or subtract.
- 4View the calculated result date instantly, including the day of the week.
- 5Toggle "Business days only" to skip weekends in your calculation.
- 6See additional information like which week of the year the result falls in.
- 7Use the quick buttons for common calculations (30 days, 90 days, 6 months).
- 8Copy the result to your clipboard for easy pasting into documents.
Date Math Fundamentals
Adding Days: Simply count forward on the calendar, including or excluding the starting day based on your needs.
- Today + 7 days = One week from now
- Today + 30 days ≈ One month from now
- Today + 365 days = One year from now (366 if crossing a leap year)
Adding Weeks: Multiply weeks by 7 and add as days.
- +1 week = +7 days
- +2 weeks = +14 days
- +52 weeks ≈ 1 year (but not exactly 365 days)
Adding Months: Add to the month number, keeping the same day of month when possible.
- January 15 + 1 month = February 15
- January 31 + 1 month = February 28 (or 29 in leap years)
- August 31 + 1 month = September 30 (September only has 30 days)
Adding Years: Add to the year number, keeping month and day the same.
- March 15, 2024 + 1 year = March 15, 2025
- February 29, 2024 + 1 year = February 28, 2025 (2025 is not a leap year)
Subtracting Time: Same rules apply in reverse.
- March 1 - 1 day = February 28 (or 29)
- March 31 - 1 month = February 28 (or 29)
Month-End Edge Cases
The "End of Month" Problem: When adding months to dates near month end, the result may need adjustment because months have different lengths.
Examples:
| Start Date | + 1 Month | Result | Why |
|---|---|---|---|
| January 28 | +1 month | February 28 | Normal |
| January 29 | +1 month | February 28/29 | Feb has fewer days |
| January 30 | +1 month | February 28/29 | Falls to last day |
| January 31 | +1 month | February 28/29 | Falls to last day |
| March 31 | +1 month | April 30 | April only has 30 days |
| May 31 | +1 month | June 30 | June only has 30 days |
Different Approaches:
- "Same day" method: Try to keep the same day number; if impossible, use last day of month
- "Last day" method: If starting on last day, end on last day of target month
Our calculator uses the "same day" method with fallback to last day of month.
Tip: For consistent results when adding months to month-end dates, consider using the 28th or earlier.
Leap Year Handling
Leap Year Rules: A year is a leap year if:
- It is divisible by 4, AND
- If divisible by 100, it must also be divisible by 400
| Year | Leap Year? | Reason |
|---|---|---|
| 2024 | Yes | Divisible by 4 |
| 2025 | No | Not divisible by 4 |
| 2028 | Yes | Divisible by 4 |
| 2100 | No | Divisible by 100 but not 400 |
| 2000 | Yes | Divisible by 400 |
February 29 Calculations:
- Adding 1 year to February 29, 2024 → February 28, 2025
- Adding 4 years to February 29, 2024 → February 29, 2028
- Subtracting 1 year from February 29, 2024 → February 28, 2023
Leap Year Frequency:
- 97 leap years every 400 years (not 100)
- Average year length: 365.2425 days
- Extra day every ~4 years keeps calendar aligned with seasons
Crossing Leap Years: When adding 365 days, be aware:
- Crossing Feb 29 → Result is 366 days later in calendar terms
- Not crossing Feb 29 → Result is exactly 365 days later
Common Date Calculations
Business Deadlines:
| Calculation | Common Use |
|---|---|
| +7 days | One week deadline |
| +14 days | Two week notice |
| +30 days | Net 30 payment, monthly review |
| +45 days | Extended notice period |
| +60 days | Net 60, some contracts |
| +90 days | Quarterly milestone, probation |
| +180 days | 6-month review |
| +365 days | Annual anniversary |
Legal and Financial:
| Term | Calculation |
|---|---|
| Net 10 | Invoice date + 10 days |
| Net 30 | Invoice date + 30 days |
| Net 60 | Invoice date + 60 days |
| Net 90 | Invoice date + 90 days |
| 30-day notice | Today + 30 days |
| 90-day warranty | Purchase date + 90 days |
| 1-year warranty | Purchase date + 1 year |
Personal Milestones:
| Event | Calculation |
|---|---|
| Pregnancy due date | LMP + 280 days (40 weeks) |
| 1st trimester end | LMP + 91 days |
| 2nd trimester end | LMP + 182 days |
| 90-day sobriety | Start date + 90 days |
| 6-month anniversary | Start date + 6 months |
Business Days Calculations
Business Days vs Calendar Days: Business days exclude weekends (Saturday and Sunday). Some calculations also exclude holidays.
Conversion Approximations:
| Business Days | Calendar Days (Approx) |
|---|---|
| 5 | 7 (1 week) |
| 10 | 14 (2 weeks) |
| 15 | 21 (3 weeks) |
| 20 | 28 (4 weeks) |
| 22 | 30 (1 month) |
| 65 | 91 (1 quarter) |
| 130 | 182 (6 months) |
| 260 | 365 (1 year) |
Quick Formula: Business days ≈ Calendar days × 5/7 Calendar days ≈ Business days × 7/5 = Business days × 1.4
Holiday Considerations:
- US has ~11 federal holidays per year
- Actual workdays per year: ~249 (260 - 11)
- Additional company holidays vary
When to Use Business Days:
- Contract deadlines
- Legal filing requirements
- Project planning
- Delivery estimates
- Response time requirements
When to Use Calendar Days:
- Age calculations
- Statute of limitations (usually)
- Calendar milestones
- Countdown timers
Day of Week Calculations
Why Day of Week Matters:
- Business deadlines on weekends may move to Monday
- Events need appropriate day scheduling
- Religious observances fall on specific days
- Recurring meetings need consistent days
Day of Week Patterns: Dates advance by one day of week each year (two during leap years).
| If Jan 1 is... | Then Jan 1 next year is... |
|---|---|
| Monday | Tuesday (Wed if leap year) |
| Tuesday | Wednesday (Thu if leap year) |
| Wednesday | Thursday (Fri if leap year) |
| ... | ... |
Doomsday Algorithm (Mental Math): Certain dates always fall on the same day each year:
- 4/4, 6/6, 8/8, 10/10, 12/12
- Last day of February
- 5/9, 9/5, 7/11, 11/7
Same Dates, Same Day: These dates share a day of week within any given year:
- January 3rd in non-leap years (January 4th in leap years)
- February 28th (February 29th in leap years)
- March 7th
Our Calculator Shows: For every calculation, we display the resulting day of week, making it easy to plan meetings, events, and deadlines.
Week Number and Calendar Weeks
ISO Week Number Standard:
- Weeks start on Monday
- Week 1 contains the first Thursday of the year
- Week numbers range from 1 to 52 (or 53)
US Convention:
- Weeks typically start on Sunday
- Week 1 begins January 1
Week Number Examples (2026):
| Date | ISO Week | Notes |
|---|---|---|
| January 1, 2026 | Week 1 | Thursday |
| January 5, 2026 | Week 2 | Monday, new week starts |
| December 28, 2026 | Week 53 | Last week of year |
Using Week Numbers:
- Project planning by weeks
- Payroll periods
- Academic calendars
- Manufacturing schedules
Quarters and Weeks:
| Quarter | Weeks (Approx) |
|---|---|
| Q1 (Jan-Mar) | Weeks 1-13 |
| Q2 (Apr-Jun) | Weeks 14-26 |
| Q3 (Jul-Sep) | Weeks 27-39 |
| Q4 (Oct-Dec) | Weeks 40-52/53 |
Our calculator displays:
- Week number (ISO standard)
- Day of the year (1-365/366)
- Quarter of the year
Historical and Future Dates
Working with Historical Dates: Our calculator supports dates going back centuries, but be aware:
Gregorian Calendar Adoption:
- Introduced in 1582
- Different countries adopted at different times
- Britain/US: 1752 (jumped from Sep 2 to Sep 14)
- Russia: 1918
- Calculations before these dates may not match historical records
Future Date Planning: The calculator handles dates far into the future:
- Retirement planning (decades ahead)
- Long-term contracts
- Generational planning
Common Long-Range Calculations:
| Purpose | Typical Range |
|---|---|
| Career planning | 10-40 years |
| Retirement | 20-50 years ahead |
| Mortgages | 15-30 years |
| Life insurance | 20-40 years |
| Infrastructure | 50-100 years |
Century Boundaries:
- 21st century: Jan 1, 2001 - Dec 31, 2100
- 22nd century: Jan 1, 2101 - Dec 31, 2200
- Year 2100 is NOT a leap year
- Year 2000 WAS a leap year
Day of Week for Famous Future Dates:
- January 1, 2030: Wednesday
- January 1, 2050: Saturday
- January 1, 2100: Friday
Age and Date Milestone Calculations
Calculating Exact Ages: Understanding exact age in years, months, and days requires accounting for varying month lengths.
Age Calculation Methods:
| Method | Description | Use Case |
|---|---|---|
| Completed years | Most common age expression | Legal age, birthdays |
| Years + months | More precise for young children | Pediatric care |
| Years + months + days | Maximum precision | Legal documents |
| Total days | Absolute measure | Scientific, actuarial |
Legal Age Milestones (US):
| Age | Milestone | Effective Date |
|---|---|---|
| 16 | Driving permit (most states) | 16th birthday |
| 18 | Legal adult, voting, military | 18th birthday |
| 21 | Legal drinking age | 21st birthday |
| 25 | Lower car insurance rates | 25th birthday |
| 26 | Off parents' health insurance | End of month of 26th birthday |
| 62 | Early Social Security | Month after 62nd birthday |
| 65 | Medicare eligibility | 3 months before 65th birthday |
| 66-67 | Full retirement age | Depends on birth year |
Anniversary Calculations:
- Paper (1 year) through Diamond (75 years)
- Work anniversaries for tenure-based benefits
- Subscription renewal dates
Pro Tips
- 💡For recurring monthly events, use the same day each month (like the 15th) to avoid end-of-month confusion.
- 💡When adding months to month-end dates, expect possible day adjustments.
- 💡Use business days for work deadlines, calendar days for personal milestones.
- 💡Remember: February 29 only exists in leap years (2024, 2028, 2032...).
- 💡For approximate month-to-day conversion: 1 month ≈ 30 days, 3 months ≈ 90 days.
- 💡Net 30/60/90 payment terms typically use calendar days, not business days.
- 💡When a deadline falls on a weekend, it often moves to Monday (check your context).
- 💡Use the week number feature for project planning and sprint scheduling.
- 💡For long-term planning, remember that leap years occur every 4 years (mostly).
- 💡Cross-check important deadline calculations manually for critical applications.
- 💡The "day of year" number (1-365) is useful for tracking progress through the year.
- 💡When in doubt about month addition, use days for precise control.
Frequently Asked Questions
Enter today's date as the start date, select "add," and enter 90 days. The calculator will show the exact date 90 days from now, including the day of the week. Remember that 90 calendar days is approximately 3 months but may not land on the same day of the month due to varying month lengths.
