Date Calculator — Add, Subtract & Days Between Dates

Add or subtract days, weeks, months, and years—or count days between two dates—with leap-year and end-of-month rules handled automatically. Calculations stay in your browser.

➕➖ Add or Subtract From a Date

Add or subtract years, months, weeks, or days from a specific date.

By Muhammad Abdullah Rauf · Founder, EverydayTools.proUpdated 2026

What is a date calculator?

A date calculator adds or subtracts days, weeks, months, or years from a reference date to find a future or past date — useful for deadlines, expiry dates, and contract timelines.

A date calculator performs calendar arithmetic that is surprisingly tricky to do by hand: adding 3 months to January 31 doesn't give April 31 (April has 30 days), and adding 1 year to February 29 only works in leap years. The calculator handles all edge cases automatically.

Date arithmetic appears in almost every professional context: contract notice periods, subscription renewals, warranty expiry, project milestones, payment due dates, and regulatory filing deadlines all require adding or subtracting a specific time period from a reference date.

Quick answers

Concise answers for common searches — definitions, steps, and comparisons.

What is end-of-month clamping in date arithmetic and how does it work?

End-of-month clamping is the rule applied when adding months would produce an invalid date. The rule: when the resulting month has fewer days than the source day-of-month, clamp to the last valid day of the target month. Examples: January 31 + 1 month → February 28 (February has 28 days, clamped from 31). January 31 + 1 month in a leap year → February 29. March 31 + 1 month → April 30 (April has 30 days, clamped from 31). May 31 + 3 months → August 31 (August has 31 days, no clamping needed). This behavior is used by banks for monthly loan payments, subscription services for monthly billing, and legal contracts with monthly notice periods.

How do you manually calculate a business days deadline (excluding weekends)?

To add N business days to a start date: (1) Start from the day after your start date (or the start date itself if including it). (2) For each business day needed, advance by 1 day. (3) If the resulting day is Saturday, advance to Monday (+2). If Sunday, advance to Monday (+1). (4) Repeat until N business days have been counted. Example: adding 5 business days to Friday January 16 → count Monday Jan 19 (1), Tuesday 20 (2), Wednesday 21 (3), Thursday 22 (4), Friday 23 (5) → deadline is January 23. Do NOT simply add 7 calendar days (that only works if your start day is a Monday). Public holidays must be excluded separately by checking a holiday calendar.

What is the difference between contract time and calendar time in date arithmetic?

Calendar time counts actual days on the calendar (including weekends and holidays). Contract time counts only specified units based on legal or business rules. The difference matters in three scenarios: (1) Notice periods — a '30-day notice' in most contracts means 30 calendar days from the day notice is given, not 30 business days. Check the contract for 'business days' or 'calendar days' language. (2) Prescription durations — '28 days' is almost always 28 calendar days. (3) Regulatory deadlines — IRS, court, and government deadlines typically mean calendar days unless explicitly stated as business days. When the deadline falls on a Saturday, Sunday, or public holiday, the due date usually extends to the next business day — this is a legal rule, not mathematical rounding.

How to use Date Calculator — Add, Subtract & Days Between Dates

  1. Enter a start date

    Select or type the start date using the date picker. This is the date you want to calculate from — today, a contract date, a deadline, or any reference point.

  2. Add or subtract time

    Choose whether to add or subtract days, weeks, months, or years. Enter the number of units you want to calculate. Useful for finding deadlines, expiry dates, or countdowns.

  3. Read the result date

    The calculator shows the resulting date including the day of the week. It accounts for leap years and varying month lengths automatically.

  4. Count days between two dates

    Use the days-between section for project spans; tick include end day when the deadline date itself counts.

Who uses Date Calculator — Add, Subtract & Days Between Dates?

Common real-world scenarios where this tool saves time.

Project deadline calculation

Add 30, 60, or 90 days to a contract start date to find payment deadlines, delivery dates, or milestone targets — accounting for weekends and month lengths automatically.

Subscription and warranty expiry

Calculate when a 12-month subscription, 2-year warranty, or 30-day free trial expires from any start date.

Legal and contract timelines

Calculate notice periods (30 days, 3 months), statute of limitations deadlines, or contractual response windows precisely.

Medical and health planning

Add a prescription duration (7 days, 28 days) to a start date to find when a course of treatment ends or when a follow-up is due.

Workflow guides

Step-by-step chains that connect related tools for common tasks.

Contract deadline: start date + notice period

  1. Enter contract start or notice date in Add/Subtract mode.
  2. Add days, weeks, or months per the clause (check calendar vs business days).
  3. Copy the resulting date and weekday into your calendar or filing system.

Date Calculator — Add, Subtract & Days Between Dates examples

90-day contract deadline

Input

Start: January 15, 2026 · Add: 90 days

Output

April 15, 2026 (Wednesday)

90 calendar days from January 15 lands on April 15. The calculator shows the day of the week too — useful for confirming deadlines don't fall on weekends.

Subscription renewal date

Input

Start: November 30, 2025 · Add: 3 months

Output

February 28, 2026

Adding 3 months to November 30 would give February 30 — which doesn't exist. The calculator correctly clamps to the last day of February (28 in a non-leap year).

28-day prescription course

Input

Start: March 3, 2026 · Add: 28 days

Output

March 31, 2026 (Tuesday)

28 days from March 3 = March 31. Use this for medication courses, follow-up appointment scheduling, and treatment timelines.

Reference tables

Date Calculator vs related tools

TaskThis toolUse instead
Add 90 days to a start dateAdd/subtract section—
Days between two datesDays-between section—
Exact age from birth date—Age Calculator
Elapsed hours between timestamps—Time Duration Calculator
Time zone for a meeting—Time Zone Converter

Calendar days vs business days

ModeCountsExample use
Calendar daysAll days including weekends90-day return window, prescriptions
Business daysMon–Fri only (no holidays)Net-30 business-day payment terms
Include end dayInclusive end date in spanContract “through” a deadline date

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Frequently Asked Questions

Can I add business days only (excluding weekends)?

The standard calculator adds calendar days. If your tool includes a business days mode, enable it to skip Saturdays and Sundays. For example, adding 5 business days to a Monday gives the following Monday, not Saturday.

How are month additions handled?

Adding months preserves the day of month when possible. Adding 1 month to January 31 gives February 28 (or 29 in leap years) because February doesn't have a 31st. Adding 1 month to March 31 gives April 30. This follows standard calendar arithmetic used by banks, legal contracts, and subscription services.

Does the calculator account for leap years?

Yes. Leap year handling is built in. Adding 1 day to February 28 in a leap year gives February 29, not March 1. Similarly, calculating the number of days between dates across a leap year gives the correct count.

How do I find days between two dates?

Enter start and end dates in the days-between section, choose whether to include the end day, and click Calculate. The tool uses exact calendar arithmetic.

Are my dates uploaded to a server?

No. Calculations run locally in your browser with JavaScript—dates are not sent to EverydayTools servers.

Privacy, accuracy, and trust

Privacy

Dates you enter are processed in your browser—they are not uploaded to EverydayTools servers.

Accuracy

Month additions use end-of-month clamping; leap years follow standard calendar rules. Business days exclude weekends only—not public holidays.

For legal, tax, or court deadlines, confirm calendar vs business-day language in your contract or jurisdiction.

Part of Date & Time Tools

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Reviewed by EverydayTools Editorial Team on 2026-05-20.