Date Calculator

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Calculate exactly how many days, weeks, months, and years are between any two dates. Includes working days.

Days Between Dates

0 days

Weeks0
Months (approx)0
Years (approx)0
Working Days0

📐 Formula

Days Between = |Date2 − Date1| ÷ 86,400,000 milliseconds. Working days ≈ Total days × (5/7)

How to Use the Date Calculator

1

Choose calculation type

Select 'Days between dates' to find the gap between two dates, or 'Add/subtract days' to find a date that is a specific number of days from a starting point.

2

Enter the start and end dates

Input dates using the date picker. For legal or contract deadlines, verify whether the start date itself counts as day 0 or day 1 — this varies by jurisdiction and contract.

3

Toggle calendar vs business days

Business days (Monday–Friday, excluding weekends) are used for legal deadlines, contract terms, and financial settlements. Calendar days count every day including weekends and holidays.

4

Note the day of the week

The result includes the weekday name — useful when scheduling around weekends or when a deadline falls on a holiday.

How Date Calculations Work

Every date corresponds to a Julian Day Number — a continuous count of days since January 1, 4713 BC. Computers subtract two Julian Day Numbers to find the exact number of days between dates, handling leap years, varying month lengths, and calendar reforms automatically. A leap year occurs every 4 years (with exceptions: century years must also be divisible by 400 — 2000 was a leap year; 1900 was not).

Business Day Calculations: Why They Differ

Legal and financial contexts frequently specify business days rather than calendar days. A 30-business-day deadline is approximately 6 calendar weeks — but the exact end date depends on which weekdays fall within the range. Most US legal rules exclude weekends but do not automatically exclude federal holidays; whether a holiday counts depends on the specific rule or contract. The Federal Rules of Civil Procedure (Rule 6) define a business day as any day that is not Saturday, Sunday, or a federal legal holiday. When accuracy matters, verify the applicable rule and consider which holidays fall within the period.

Common Date Calculation Use Cases

Contract expiry and notice periods; court filing deadlines; age verification for legal purposes (e.g. is this person 18 today?); project timelines and milestone planning; warranty and insurance policy durations; payroll and billing cycles; calculating days until an event; and determining how many days of a lease or subscription have elapsed. For legal matters, always confirm the counting method — inclusive vs exclusive of the start date, and calendar vs business days — before relying on any date calculation tool.

How to Calculate Dates by Hand: Worked Example

Adding calendar days: starting from July 6, 2026 and adding 90 days lands on October 4, 2026 — simply counting forward day by day through July (25 remaining days), August (31), September (30), and 4 days into October: 25+31+30+4 = 90.

Counting business days: between July 6, 2026 and September 30, 2026 there are 86 total calendar days, but only 62 business days once weekends are excluded — roughly a 28% reduction, since a typical 12-week span includes about 24 weekend days. This gap is why a "90-day" contractual deadline and a "90 business day" deadline can land nearly six weeks apart.

Why do calendar-day and business-day deadlines diverge so much over long spans?

Every 7 calendar days contains exactly 5 business days, so the ratio compounds: over 90 calendar days (about 12.9 weeks), roughly 25–26 of those days are weekends, leaving around 64–65 business days — closely matching the 62 calculated above once specific weekday alignment is accounted for. Contracts, court filings, and payment terms that specify "business days" rather than "calendar days" are effectively extending the deadline by nearly a third.

What Are the Most Common Real-World Uses for Date Math?

How is a project deadline calculated from a start date?

Project planning typically adds a duration in business days to a start date, explicitly skipping weekends and, for precise planning, public holidays. A project starting July 6, 2026 with a 60-business-day timeline lands in early October — meaningfully later than a naive 60-calendar-day count (which would land in early September), a common source of missed-deadline surprises when teams don't distinguish the two.

How do you calculate someone's exact age in days for a legal or medical context?

Subtract the birth date from the current date directly — the same method used in the age calculator above — which for legal contexts (age of majority, eligibility cutoffs tied to an exact date) is more precise than counting whole years, since eligibility can hinge on a single day's difference around a cutoff date.

Why does adding "1 month" to a date sometimes behave unexpectedly?

Adding one calendar month to January 31 has no exact equivalent — February has only 28 or 29 days. Most date systems resolve this by capping the result at the last valid day of the target month (giving February 28 or 29), which can silently shift a monthly billing or recurrence date by a day or two compared to naive day-counting, especially across February in non-leap years.

Frequently Asked Questions

Subtract the earlier date from the later date in milliseconds, then divide by 86,400,000 (milliseconds per day). This calculator handles all the complexity for you.
A standard year has 260–262 working days (52 weeks × 5 days), minus public holidays. In the US, with 10 federal holidays, the typical number is about 250 working days per year.
Calendar days count every day including weekends and holidays. Working days (business days) only count Monday–Friday, excluding public holidays. Always clarify which type is meant in contracts or deadlines.
Enter today's date and your target date in the calculator above. The result accounts for leap years automatically. Common uses: days until a deadline, event, or expiration. The US averages 261 working days per year after excluding weekends — the calculator can show business days only if needed.
There are exactly 52 weeks and 1 day in a standard year (365 days), or 52 weeks and 2 days in a leap year (366 days). This is why your birthday advances by one day of the week each year (two days after a leap year). For project planning, most businesses use 52 weeks as the working year.

Sources & Methodology

Calculations are based on the most current publicly available data from authoritative government and industry sources: