Date Calculator: Add Days, Subtract Dates, and Find Weekdays

How to utilize this calendar

Change the time on a date

• Use the date picker to choose your start date.
• Type in the number of days, weeks, months, or years
• Pick "Add" or "Subtract"
• Click "Calculate" to see what you got.

Find the day of the week for any date

• Type in any date, whether it's in the past or the future
• The calculator shows the day of the week right away

Countdown to a target date

• Put in the date you want to reach (event, deadline, holiday)
• See how many days are left exactly

Learning about your results

Results are shown in UTC (Coordinated Universal Time) so that they are the same for everyone throughout the world. You'll see the computed date, the day of the week, and the total time in many ways, such as days, weeks, months, and years. All calculations automatically take leap years into account.

What is a date calculator, and when should you use one?

A date calculator is a tool that lets you do math with dates, including adding, subtracting, or computing the difference between two dates. Date calculations are more complicated than simple math since they have to take into account leap years, the fact that months can have different numbers of days (28 to 31), and the regulations of the calendar system.

You need a date calculator anytime counting by hand is too hard. Project managers utilize them to set timelines that are possible. Parents-to-be figure out when their baby is due. People who are going on a trip count down the days till they leave. Lawyers set the deadlines for filing. Anyone who keeps track of warranties, subscriptions, or contracts needs to be good at math with dates.

The problem with dates is that they aren't always the same. February has 28 days, but in leap years, it has 29. Adding "one month" to January 31 presents questions because there is no February 31. Using normal Gregorian calendar principles, our calculator covers these edge scenarios, so there is no guesswork or mistakes.

How date computations really work in the background

Modern date calculators turn calendar dates into serial numbers, which are integers that show how many days have passed since a certain point in time. For example, January 1, 2020, could be serial number 43831, while December 31, 2020, could be 44196. When you take these numbers away from each other, you get 365 days (or 366 in leap years).

Adding days is easy: just increase the serial number and change it back to a calendar date. The calculation goes from one month to the next and one year to the next without any help from the user.

Adding months is more complicated. When possible, the calculator keeps the day number. For example, March 15 + one month = April 15. But if you add one month to January 31, you get February 28 (or 29), since February doesn't have a 31st day. This change at the end of the month is in line with normal calendar rules.

Adding years works in the same way. If someone was born on February 29, 2020, and they add one year, they will be born on February 28, 2021, because 2021 is not a leap year. Adding four years brings them back to February 29, 2024.

You probably learned the leap year rule improperly.

Most people know that leap years happen every four years. That's not enough. The Gregorian calendar utilizes a more accurate algorithm to keep our calendar in sync with the orbit of the Earth.

A year is a leap year if it can be divided by 4, however years that end in 100 (1900, 2100) must also be able to be divided by 400. This is why 1900 wasn't a leap year but 2000 was.

This rule makes the average year 365.2425 days long, which is quite near to the actual length of Earth's orbit, which is 365.2422 days. The Gregorian calendar only moves one day every 3,030 years, which is much better than the Julian calendar, which moves one day every 128 years.

Examples of leap years

• 2024: A leap year of 366 days, it can be divided by 4.
• 2025: Not divisible by 4, a common year with 365 days
• 2100: Divisible by 4 and 100, but not by 400. Not a leap year.
• 2400: Can be divided by 400, which means it's a leap year.

A step-by-step tutorial to figuring out dates by hand

Our calculator gives you answers right away, but knowing how to do math by hand gives you more faith in the answers.

How to find the number of days between two dates

• First, figure out how many days are left in the start month. If you start on March 15, there are 31 days left in March, so 16 days are left (31 - 15 = 16).
• Next, put full months between the start and end dates. There are 91 days in total: 30 in April, 31 in May, and 30 in June.
• Then add the number of days that have passed in the last month. Add 20 days if it ends on July 20.
• Add everything up: 16 + 91 + 20 = 127 days.

To find a date in the future, add days.

Convert your desired days into months and days left. Adding 100 days to January 1: January has 31 days (31 used), February has 28 days (59 total), March has 31 days (90 total), and April has 10 days left. The result is April 10.

How to figure out when your baby is due

Add 280 days (40 weeks) to the first day of your last menstruation to get the date of your next period. You could also take 3 months off the LMP, add a year, and then add 7 days.

Example of how to work on a project by the deadline

Your project starts on January 15, 2025, and you have 90 days to finish it.

• January: 16 days left (31 - 15)
• February: 28 days (44 days total)
• March: 31 days (75 days total)
• April: 15 more days are needed (90 total)

Due date: Tuesday, April 15, 2025

Common errors that cause inaccurate date calculations

Using the wrong date format can lead to expensive mistakes around the world. In the US, the date 03/04/2025 means March 4, but in Europe, it implies April 3. Use the ISO 8601 format (YYYY-MM-DD) or write down the month name when accuracy is important.

Not taking leap years into account messes up computations that span more than one year. Someone who thinks there are 365 days in a year makes a mistake every four years, which adds up to big mistakes over decades.

Counting inclusive and exclusive dates wrong leads to errors that are off by one. Does "within 30 days" mean the start date? Contracts and legal papers usually say, but everyday use is different.

Thinking that all months have 30 days is too simple. For keeping track of deadlines accurately, the difference between February with 28 days and March with 31 days is important.

Not remembering that time zones are important for working together across borders. In London, it's already January 1 at 11 PM on December 31. Our results based on UTC are the same no matter where you are.

Things to think about in our worldwide audience's region

In many parts of the world, the way dates are written is different. In the United States, MM/DD/YYYY is the most common format. DD/MM/YYYY is the most common format in Europe, South America, and most of Asia. The international standard is YYYY-MM-DD (ISO 8601). It is utilized in science, computing, and countries like China and Japan.

You can enter dates into this calculator in any common format, and it will show you the results properly so there is no confusion.

The definitions of a work week also differ around the world. Most Western countries have a work week from Monday to Friday, whereas several Middle Eastern countries have a work week from Sunday to Thursday. In some parts of Germany and Latin America, Saturday is still a half-day of labor.

Days on the calendar, days at work, and days off

Calendar days tally every day on the calendar, which is 365 or 366 days a year. This covers every day, even weekends and holidays.

Business days, which are sometimes termed working days, are usually Monday through Friday, without including weekends. This gives you about 260 business days a year in most nations, not counting holidays.

In certain cases, working days and business days are the same, but in other others, they may not include public holidays. The difference is important for shipping estimates, payment conditions, and legal deadlines.

Quick conversion: About 14 calendar days are equal to 10 business days. To change business days into calendar days, multiply by 1.4.