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Date Calculator
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💡 Expert Tips
Business Days vs Calendar Days
Contracts often say "30 business days." That's NOT the same as 30 calendar days. Business days skip weekends. So 30 business days is actually about 42 calendar days (6 weeks). Use this calculator for calendar days, then add 40% if you need business days.
The Inclusive vs Exclusive Thing
Some people count "both days." Some don't. If you're booking a hotel from Friday to Sunday, that's 2 nights (not 3). But if you're counting "days since birth," you usually include your birthday. This calculator counts the days between (exclusive). Add 1 if you need inclusive.
Leap Years Are Weird
Every 4 years, February gets an extra day. Except years divisible by 100 (like 1900). Except years divisible by 400 (like 2000). Yeah, it's confusing. The calculator handles this automatically, but just know that "365 days" isn't always a year.
⚠️ Common Mistakes
Mixing Up Start and End Dates
The order matters for some calculations. If you're adding 30 days to November 15, you get December 15. But if you flip them, you're subtracting. Double-check which date is your starting point.
Forgetting Time Zones
This calculator uses dates, not times. So if you're born at 11:59 PM on Dec 31, 2000, and it's 12:01 AM on Jan 1, 2025, you're still 24 years old (not 25). The calculator doesn't care about hours and minutes.
Using Wrong Month Lengths
"3 months from January 31" is NOT April 31 (that date doesn't exist). The correct answer is May 1 or May 2, depending on how you count. This calculator avoids months when possible – it uses days instead.
📖 Understanding Date Calculations
Need to know how many days until your vacation? Or how long ago something happened? Pick two dates. Calculator shows the difference in days, weeks, and months. Takes 10 seconds.
There are two modes: calculate the gap between dates, or add/subtract days from a date. Both are useful for different things.
Mode 1: Days Between Dates
This calculates the difference between two dates. Common uses:
- Age calculations – How many days have you been alive?
- Event countdowns – Days until Christmas, your wedding, etc.
- Contract durations – How long was that lease?
- Historical gaps – Days between two historical events
Example: From January 1, 2024 to November 24, 2025
= 693 days (or about 99 weeks, or 23 months)
Mode 2: Add/Subtract Days
Start with a date, then add or subtract days to get a new date. Common uses:
- Deadlines – What's the date 90 days from now?
- Return dates – When's 30 days before a trip?
- Payment schedules – Invoice due in 45 days
- Project timelines – Add 120 days to a start date
Example: Today is November 24, 2025. Add 90 days.
= February 22, 2026
Why Months Are Approximate
Months aren't all the same length. February has 28 or 29 days. Most other months have 30 or 31. So when the calculator says "≈ 6.2 months," that's an average. We use 30.44 days per month (365.25 ÷ 12).
If you need exact calendar months (like "3 months from today"), you have to count manually or use your calendar app. This calculator focuses on days because they're exact.
Real-World Examples
- Pregnancy tracking – 40 weeks = 280 days from last period
- Statute of limitations – Legal cases often have 365-day or 730-day deadlines
- Product returns – "90-day return policy" means 90 calendar days
- Loan maturity – "6-month CD" usually means 180-183 days
- Subscription renewals – Annual = 365 days (or 366 in leap years)