Use this days calculator to easily add or subtract days to any given initial date. **Add days to date** and see what date it will be *N* days from now, or **what date it was N days ago** using this day adder. Also shows what day of the week it will be.

Quick navigation:

- How to add days to a date
- Practical applications of adding days
- What day will it be in X days?
- History of timekeeping and calendars

This add days calculator is a simple tool you can use when you need to add or subtract days to/from a specific date; or in other words, to see which date it will be after N days have passed, or which date it was N days before any given date. The date format adjusts automatically depending on your browser locale settings.

## How to add days to a date

If you wish to find out what date it will be 30 days from now, you just need to write the number '30' in the second field. If the date is different from the current one, let's say Feb 20, 2019, simply write it in the first field. Then click on 'Calculate'. In this case the result would be Mar 22, 2019. Note that the same calculation in the year 2020 will yield Mar 21, 2020 since it is a leap year and February has 29 days.

Despite its name, you can also use the add days calculator to *subtract*, for example, 90 days from a specific date. First, make sure the right date is entered in the date field and then write '-90' in the second field. If we subtract 90 days from Feb 20, 2019 the result would be Nov 22, 2018.

With both days addition and subtraction, if the time interval is expressed in terms of weeks, months, years, or decades, it is first necessary transform these into a number of days before performing the operation. It is easy to do with weeks (1 week = 7 days), but the other time measures have variable length so the exact number of days will vary depending on the calendar.

## Practical applications of adding days

Adding days is useful in many everyday situations. Let's say you ordered custom furniture and you were given a 45-day completion term. With this calculator for adding days, you can easily see when you can expect your furniture to be ready. Just add 45 days to the current date. If the time was given in business days, make sure to use our working days calculator instead!

Another common case when you need to add days is when you want to **return a purchase** or make use of a **guarantee** for a given product. Say you have a 14-day free return period and you know you made the purchase on Dec 20, 2017. You enter this date in the date field, then enter 14 in the "days to add" field and the day adder will tell you that you have until end of day on Jan 3, 2018 to return the product.

In another example, maybe you were **prescribed medication** to take for 10 days, after which you need to visit your GP for evaluation of the treatment. Just enter the date you started taking your medication in the "Date" field and 10 in the "days to add" field of the add days calculator to learn the date after which you should go see your physician again.

## What day will it be in X days?

A typical use of a day adder is to check what day of the week it will be in a given number of days. For example, to see what day it will be in 10 days, enter "10" in the "Days to add" field before pressing "Calculate". You see both the resulting date, and the day of the week (Sunday through Monday) it will be. Likewise you can check what day it will be in 5 days, 20 days, or any number of dates.

A trick you can use to get the answer by mental calculation is to check if the number of days to add is wholly divisible by seven. If it is, then it will be exactly the same day of the week. For example, if the question is "what day will it be in 7 days" and it is Tuesday today, it will be Tuesday in seven days as well. Likewise it will be Tuesday in 14, 21, or 28 days.

## History of timekeeping and calendars

Knowing the calendar is crucial for adding days correctly. A calendar is a timekeeping system which is made for civil, religious, or administrative purposes. Calendars are usually based on astronomical events, such as the movement of the Earth and the Moon. The word calendar could also be used as a reference to a device, usually paper, which shows the dates in a particular period of time; or to a time table for events, such as cultural ones.

Methods for timekeeping have existed as far back as the Neolithic period; thousands of years BC ^{[1]}. For obvious reasons the* day* is the most natural unit to use: one of the most obvious things for human beings is that there is darkness and brightness at about equal periods of time, which affect the way we live, sleep, eat, etc. Other than the day, there is obviously *the solar year* which dictates the seasons; from a human standpoint, this is important for growing and finding food, and overall surviving. Last but not least, there are the *lunar phases*; they are quite obvious for the naked eye and are of great importance in some religions and in many myths.

Different calendars warrant a different way to add days to a given date. **Solar calendars** are one of the two main kinds. The first solar calendar is likely the Egyptian, which might have been developed as early as 4000 BC. A solar calendar is based on the change in the position of planet Earth in regards to the Sun. In a solar calendar one year represents one complete rotation of Earth around the Sun. There are two types of solar calendars: *Tropical* and *Sidereal*.

The Gregorian calendar, which is currently the most widely used calendar in the world, is solar, thus, it is not related to the moon cycles. The mean length of its year is 365.25 days. To make it dividable into days we accept that three years in a row will have 365 days and the forth one, also called *leap year*, will have 366 days with one more day added to February. This is the calendar used in our calculator.

**Lunar calendars** are based on the movement of the Moon around the Earth. They use the lunar month, also called a lunation. This is the period between two new moons, equal to about 29.5 days. A month in a lunar calendar is either 29 or 30 days, as opposed to solar months which range between 28 and 31 days. The brightest example of such calendar would the Islamic, currently the official one in Saudi Arabia and other countries.

The Islamic calendar does not correspond to the movement of the Sun and thus the change in seasons, which can make it quite difficult to work with if you are in agriculture. Because the lunar year is about 11 days shorter than the solar, there are big changes in when the start of January would be in relation to the seasons: one year it might be in the middle of the winter and some years later it will be in the middle of the summer. Thus, you never know on what date a certain holiday or religious celebration would be. Most calendars that have their months base on the moon cycles are actually lunisolar (or solilunar). Lunar calendars are not supported for our calculator.

**Lunisolar calendars** are a combination of the lunar and the solar. Such is the *Jewish calendar*, which is based on both the lunar cycles and the solar year. In it the beginning of the month will always be the new moon. However, every few years it adds a leap month in order to correct for seasonal changes.

Currently, our add days to date calculator only supports the internationally recognized calendar and does not support these more esoteric ones, but let us know if that is something you would like to see us do!

#### References

1 "History of calendars". [online] Available at: https://en.wikipedia.org/wiki/History_of_calendars