Date Calculator

A date calculator saves you from counting days on your fingers, flipping through calendars, or getting the arithmetic wrong when months have different lengths. This one handles five separate calculations from a single page: date difference, add or subtract time from a date, age calculation, business day counting, and day of the week finder.

Date Difference – Days Between Two Dates

Use this tab when you need the exact gap between two dates.

Pick a start date and an end date, then click Calculate. The result shows the total in days, weeks, months, and years all at once.

This covers more situations than most people expect: project timelines, subscription periods, warranty tracking, lease durations, time since a diagnosis, days until a deadline. Any time you need to know how long something has been running or how long until something ends.

Start Date	End Date	Days	Weeks	Months	Years
1 Jan 2025	31 Dec 2025	364	52.00	11.95	0.997
15 Mar 2023	15 Mar 2026	1,096	156.57	36.00	3.00
1 Jun 2025	1 Sep 2025	92	13.14	3.02	0.252
10 Oct 2024	10 Jan 2025	92	13.14	3.00	0.252

If the end date is before the start date, the calculator flags it. Same-day inputs return zero across all fields.

Add or Subtract Time from a Date

Use this tab when you know a date and need to find another date a set amount of time away.

Pick your start date, choose add or subtract, enter a number, and select the unit: days, weeks, months, or years. The result shows the new date, the day of the week it falls on, and how many days were moved in total.

Common uses: calculating when a 90-day return window closes, finding the date 6 weeks after surgery, working out when a 3-month probation period ends, or finding what date falls 180 days from today.

Start Date	Operation	Result Date	Day of Week
1 Jan 2026	Add 90 days	1 Apr 2026	Wednesday
15 Feb 2026	Add 6 weeks	29 Mar 2026	Sunday
10 Mar 2026	Subtract 3 months	10 Dec 2025	Wednesday
1 Jun 2025	Add 2 years	1 Jun 2027	Tuesday

When you add months, the calculator uses calendar months, not 30-day blocks. Adding 1 month to January 31 gives February 28 or 29 depending on the year. This matches how contracts and legal documents define months. If you need exactly 31 days rather than one calendar month, use the Days unit instead.

Age Calculator

Enter a date of birth and an “age as of” date. The age-as-of field defaults to today so you can calculate a current age without changing anything.

The result gives the exact age broken into years, months, and days. It also shows total days lived, total weeks, and the date and countdown to the next birthday.

The years, months, days breakdown is more accurate than simple division. It accounts for the varying length of months and correctly handles birthdays at the end of February in leap years.

Date of Birth	Age As Of	Exact Age	Next Birthday
15 Mar 1990	15 Mar 2026	36 years, 0 months, 0 days	365 days
1 Jan 2000	1 May 2026	26 years, 4 months, 0 days	245 days
29 Feb 2000	1 Mar 2026	26 years, 0 months, 1 day	364 days
10 Aug 1985	1 May 2026	40 years, 8 months, 21 days	101 days

The leap year example (29 Feb 2000) is handled correctly. The age-as-of field accepts any date, so you can calculate how old someone will be on a future date, or verify an age on a historical date.

Business Days Calculator

This tab counts working days between two dates, excluding weekends. It also lets you enter specific public holidays to exclude.

Enter your start date, end date, and optionally a comma-separated list of holiday dates in YYYY-MM-DD format. The calculator returns the number of business days, weekend days, and holidays excluded separately.

This matters because most contracts, legal deadlines, payment terms, and HR policies specify business days, not calendar days. “Payment due in 10 business days” is not the same as “payment due in 10 days.” A 10-business-day period starting on a Monday ends the following Friday, not 10 calendar days later.

Start Date	End Date	Calendar Days	Business Days	Weekend Days
Mon 6 Jan 2025	Fri 10 Jan 2025	5	5	0
Mon 6 Jan 2025	Sun 12 Jan 2025	7	5	2
1 Apr 2025	30 Apr 2025	30	22	8
1 Jan 2025	31 Mar 2025	90	63	26

To exclude a public holiday, add its date to the holidays field. For example, entering 2025-12-25 removes Christmas Day from the business day count. You can add multiple dates separated by commas. The result shows exactly how many holidays were excluded so you can verify the number.

Day of the Week Finder

Enter any date and the calculator returns the full day name, whether it is a weekday or weekend, the day of the year (e.g. day 92 of 365), the week number, which quarter it falls in, and whether it is a leap year.

The day name appears as a large badge. Weekdays show in dark ink. Weekends show in terracotta so you can tell at a glance without reading the label.

Date	Day	Type	Day of Year	Week	Quarter
1 Jan 2026	Thursday	Weekday	Day 1 of 365	Week 1	Q1
25 Dec 2025	Thursday	Weekday	Day 359 of 365	Week 52	Q4
14 Feb 2026	Saturday	Weekend	Day 45 of 365	Week 7	Q1
29 Feb 2028	Tuesday	Weekday	Day 60 of 366	Week 9	Q1
4 Jul 2026	Saturday	Weekend	Day 185 of 365	Week 27	Q3

The 29 Feb 2028 example confirms the leap year check. 2028 is divisible by 4 and not a century year, so it is a leap year. The result shows 366 days in the year accordingly.

This tab is useful when planning events around weekdays, checking whether a specific date falls in a particular quarter, or verifying historical dates.

Calendar Days vs Business Days Explained

This distinction comes up constantly in contracts, shipping estimates, and legal deadlines. Getting it wrong can cost you days you thought you had.

Calendar days count every day on the calendar without exception: weekends, public holidays, everything. If a contract gives you 10 calendar days starting on a Thursday, you count straight through the weekend, and your deadline lands on the Sunday after next. No days are skipped.

Business days count only Monday through Friday, excluding public holidays you specify. A 10-business-day period starting on a Thursday gives you two full weeks, because two weekends get skipped.

One critical default: in most legal and commercial contexts, a contract that says “days” without specifying means calendar days. Courts in most jurisdictions apply this default. If the drafters intended business days, they should have said so. When you see an unqualified “days” in a document and you are not sure, counting calendar days and treating that as your hard deadline is the safer approach.

There are approximately 250 to 252 business days per year in the US, depending on how federal holidays fall on the calendar. In 2026 there are 250.

Scenario	Calendar Days	Business Days
10-day period starting Monday	Ends Wednesday next week	Ends Friday this week
30-day return window starting Jan 1	Ends Jan 31	Ends approx. Feb 10-12
90-day notice period starting Jan 1	Ends Apr 1	Ends approx. Apr 30
One full year	365 or 366 days	250-252 days

Common Date Calculation Mistakes

Calendar months are not 30 days. January has 31, February has 28 or 29, and the rest vary. If you need exactly 90 calendar days rather than 3 months, use the Add/Subtract tab with days as the unit, not months. Three calendar months from January 31 lands on April 30. Ninety days from January 31 lands on May 1. That is one day apart, but it matters when a contract or deadline is involved.

Business days and calendar days are not the same. A 10-business-day window that starts on a Monday ends on the following Friday, not 10 days later. Use the Business Days tab any time you see “working days” or “business days” in a document.

Inclusive vs exclusive counting. Most date calculations treat the start date as Day 0. The result is the number of full days between the two dates. Some legal contexts count the start date as Day 1. If your calculation needs to match a specific legal or contractual definition, check which method applies before acting on the number.

“Days” in a contract usually means calendar days. This is one of the most common deadline errors. A 10-day notice period in a contract means 10 calendar days in most jurisdictions unless the contract explicitly says business days. Do not assume weekends are excluded unless the document says so.

Frequently Asked Questions

How do I calculate the number of days between two dates?

Use the Date Difference tab. Enter the start date and end date and click Calculate. The result shows the total in days, weeks, months, and years. The days result is always a whole number. Weeks, months, and years show as decimals.

What is the difference between calendar days and business days?

Calendar days count every day including weekends and public holidays. Business days count only Monday through Friday, excluding public holidays you specify. Most financial, legal, and HR deadlines use business days. In most legal contexts, a contract that says “days” without further specification means calendar days.

How does the calculator handle leap years?

The calculator uses the standard Gregorian leap year rule: a year is a leap year if it is divisible by 4, except for century years which must be divisible by 400. 2024 is a leap year. 2100 will not be. This affects February dates and any calculation that spans February 29.

Can I find out what day of the week a future date falls on?

Yes. Use the Day Finder tab. Enter any past, present, or future date and the calculator returns the day of the week, weekday or weekend status, week number, day of year, and quarter.

How do I calculate when a 90-day deadline ends?

Use the Add/Subtract tab. Enter your start date, select Add, enter 90, and select Days. The result is the date 90 calendar days later. If you need 90 business days instead, use the Business Days tab.

How do I exclude public holidays from the business day count?

In the Business Days tab, enter the holiday dates in the Holidays field, separated by commas. Use the format YYYY-MM-DD. For example: 2025-12-25, 2026-01-01. The calculator removes those dates from the business day count and shows how many holidays were excluded.

What format should I enter dates in?

Use the date picker built into each input field. Click the field and a calendar opens. You can also type the date directly. The accepted format follows your browser’s locale settings, typically DD/MM/YYYY or MM/DD/YYYY depending on your region.

Why does adding one month to January 31 give February 28?

Because February only has 28 days in a standard year (29 in a leap year). When you add one calendar month to January 31, the result is the last valid day in February. This is the correct behaviour and matches how contracts and legal documents define months. If you need exactly 31 days rather than one calendar month, use the Days unit in the Add/Subtract tab.

What does “days” mean in a contract – calendar or business days?

In most legal and commercial contexts, “days” without further specification means calendar days. This is the default applied by courts in most jurisdictions. If the contract intended to exclude weekends, it should say “business days” explicitly. When the document is ambiguous, count calendar days and treat that as your hard deadline.

How many business days are in a year?

Approximately 250 to 252 business days per year in the US, depending on how federal holidays fall on the calendar. A standard year has 365 days, minus 104 weekend days (52 weeks times 2), minus 10 to 11 federal holidays. In 2026 there are 250 business days.

Can I calculate how many days until a future event?

Yes. Use the Date Difference tab. Set the start date to today and the end date to your target event. The result shows the days, weeks, months, and years remaining. For birthdays, use the Age Calculator tab instead, which also shows the countdown to the next birthday directly.