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📅 Math Date Calculator
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Date Calculator

Days between dates, add/subtract days, business days and age calculator

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Calendar Days
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Between the two dates
Weeks and days—
Months (approximate)—
Years (approximate)—
Hours—
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days
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Result Date
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New date
Start date—
Days added/subtracted—
Day of week—
Weeks added/subtracted—
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Business Days
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Weekdays only (Mon–Fri)
Calendar days—
Weekend days—
Business weeks—
% of days that are business days—
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Age
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years old
Exact age—
Total months lived—
Total days lived—
Next birthday—
Days until next birthday—

How to Calculate Days Between Dates and Other Date Math

Date calculations are needed for everything from project deadlines to contract terms, from counting how many working days until a vacation to figuring out how long until a subscription renews. This calculator handles all four common date math problems with precision, including a business days counter that correctly excludes weekends.

Lawyers count calendar days for contract deadlines, HR teams count business days for notice periods, parents track days until events, and developers use date arithmetic constantly. All four modes update instantly as you change any input.

How Days Between Dates Works

The difference in days = (End date − Start date) / 86,400,000 milliseconds. JavaScript's Date objects handle leap years, month lengths, and daylight saving time automatically. The result is the exact number of calendar days between the two dates.

Business Days Formula

Iterate through each calendar day in the range. If the day of the week is Monday (1) through Friday (5), count it as a business day. Saturdays (6) and Sundays (0) are excluded. For a 14-day range starting on a Monday: 10 business days (2 weekends × 2 days).

When to Use This Calculator

Date math problems come up constantly in professional and personal life. This calculator handles all four with precision:

  • Contract deadlines: A freelance contract requires delivery "30 business days" after signing on March 18, 2026 — use the Business Days tab to find the exact due date (May 1, 2026, assuming no holidays).
  • Project planning: A product launch is set for June 30. How many working days does the team have starting March 18? Use Days Between to calculate the sprint length.
  • Subscription and warranty tracking: A 90-day free trial started January 5 — use Add Days to find the exact expiry date (April 5).
  • Age verification: Legal applications, insurance forms, and HR systems often require exact age in years, months, and days — use the Age Calculator tab.
  • Real estate and legal timelines: Mortgage closings, option periods, and lease expirations often specify exact calendar day counts.

Step-by-Step: Business Days Between Two Dates

From March 18, 2026 (Wednesday) to April 30, 2026 (Thursday):

Step 1 — Calendar days: 43 total days
Step 2 — Full weeks: 43 ÷ 7 = 6 complete weeks + 1 day
Step 3 — Weekends: 6 × 2 = 12 weekend days
Step 4 — Remaining day (the +1): check if it lands on a weekday — Thursday = yes
Step 5 — Business days: 43 − 12 = 31 business days

Subtract any public holidays that fall within the range for precise working-day counts used in legal and HR contexts.

Frequently Asked Questions

How many days are there between two dates? ▼
Subtract the start date from the end date. In practice, convert both dates to milliseconds, find the difference, and divide by 86,400,000 (ms per day). This accounts for leap years and variable month lengths automatically. Use the "Days Between Dates" tab above and enter any two dates for an instant answer.
How do I count only business days between two dates? ▼
Step through each calendar day in the range and count only Monday–Friday. Skip Saturdays and Sundays. If public holidays matter for your use case, subtract them manually. A rough estimate: business days ≈ calendar days × (5/7). For 30 calendar days: approximately 21 business days. Our calculator gives you the exact count.
What date is 90 days from today? ▼
Use the "Add / Subtract Days" tab: enter today's date, type 90, and select "Add". The result is your exact date 90 days from now. Since months have different lengths, you can't just add 3 months — 90 days is precise. Common use cases: trial period end dates, invoice payment deadlines, and contract terms.
How do I calculate exact age in years, months and days? ▼
Start with complete years between birth date and target date. Then count remaining complete months. Then count remaining days. This is more complex than it sounds because month lengths vary. Use the Age Calculator tab — enter birth date and target date for exact years, months, and days, plus total days lived.
What is the difference between calendar days and business days? ▼
Calendar days count every day including weekends and holidays. Business days (working days) count only Monday–Friday. A 30-day month has approximately 21–23 business days depending on which day of the week it starts. Legal contracts specify which type to use — always clarify this in agreements, as "30 business days" is very different from "30 calendar days."
How many days is 6 months? ▼
6 calendar months is approximately 181–184 days, depending on which months (leap years affect February). For precise calculations, always use specific start and end dates. The rough approximation is 365 / 2 = 182.5 days. Use the "Days Between Dates" tab to find the exact count for any 6-month period.
How do I calculate a deadline that falls on a weekend or holiday? ▼
Legal and financial deadlines that fall on a weekend or public holiday typically roll forward to the next business day. Courts, banks, and government agencies follow this rule universally. Example: a 30-day notice period ending on Saturday becomes due the following Monday. Always check jurisdiction-specific rules for contracts and legal filings — some documents specify "calendar days" explicitly to prevent roll-forward adjustments. When in doubt, treat the nearest prior business day as the safe deadline.
What is a fiscal year and how do fiscal quarters work? ▼
A fiscal year is any 12-month accounting period a company or government uses — it doesn't have to align with the calendar year. The US federal government's fiscal year runs October 1 – September 30. Many retailers end their fiscal year in January to include the holiday shopping season in Q4. Fiscal quarters divide the fiscal year into four 13-week periods. When comparing dates across fiscal years, use a date calculator to convert between fiscal and calendar periods to avoid confusion in financial planning.
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