Calendar Calculator
Use the calendar calculator to add days to date and subtract days from a date. This time calculator works similarly to the date to date calculator and the day counter, but is specifically designed for time addition and time subtraction, not counting the days between two dates. This tool works in all sorts of different time units  years, months, weeks, and days, so you can not only answer questions like "what day will be 90 days from today?" or "what date will be 6 months from today?", but also add and subtract the sum of different time units (e.g. 2 years, 6 months, 3 weeks, and 2 days). You no longer need to stress about when your deadlines are.
You are now freed from the burden of forecasting the calendar days by yourself. Rejoice.
How to use the calendar calculator

Input the start date  choose it from the calendar or type in the first three letters of the month's name followed by the day number, a comma, and the year (e.g. Oct 3, 2020).

In the next field, choose whether you want to add to or subtract time from the start date.

Input the number of years, months, weeks, and days you want to add or subtract.

The end date is the result of time addition or subtraction. If you click the calendar icon on the right to the date, you can check which day of the week it will be/was.
Add days to date using the calendar calculator  time addition example
Let's say you want to know the date 90 days from today  this means you need to add days to the date. To do that:

Choose today as the start date (it will be highlighted in the calendar).

Leave the default "add" option in the second field.

Input "90" in the "days" field. This is the number of days between the dates.

In the last field in the calendar days calculator, you'll find the date which will be 90 days from today.
Subtract days from date  time subtraction example
Let's say you need to know which day was 6 weeks and 3 days ago. To check this:

Choose today as the start date.

Choose "subtract" in the second field of the calendar calculator.

Input "6" in the "weeks" field.

Input "3" in the "days" field.

The result of your time subtraction is shown in the last field of the date calculator.
This subtraction means going back in time 45 days from today, so you could also input 45 days instead of 6 weeks and 3 days.
How the calendar date calculator works
This time calculator is based on the Gregorian calendar. This means we assume:

A year is 12 months long  that's because there are over 12 lunar cycles in a year and calendars used to be based on the motion of the Moon and the Sun.

A year is 365.25 days long  this is an approximation of the time it takes Earth to complete a full orbit around the Sun.

A week is 7 days long  this can't be explained in terms of natural phenomena. Historians suppose ancient Babylonians introduced a 7day week. The reason remains unclear; one hypothesis says they attached some spiritual meaning to number 7.
In this date calculator, the month's length is 30.44  the averaged result of diving the number of days in a year by the number of months in a year:
365.25 / 12 = 30.4375
30.4375 ≈ 30.44
The history of timekeeping devices
We've come a long way from the Big Bang to the calendar date calculator. Here are some crucial steps humanity has taken:

Sexagesimal system  a numeral system with 60 as a base (as opposed to 2 in binary and 10 in the decimal system) developed by the ancient Sumerians. Although not a timekeeping device per se, the system is significant as it is still used for calculating time (e.g. 60 seconds make 1 minute), angles, and geographical coordinates. Interestingly, seconds are called such because they are the second division of an hour, after a minute.

Sundials  one of the earliest timekeeping devices, a sundial relies on the Sun. Cloudy days and nights impede its use.

Water clock  developed in ancient Egypt, it consists of water flowing into a container with marked lines of measurement, which would inform you how much time had passed. In 4^{th} BC the Greeks started using it and called it clepsydra ("water thief").

Candle clock  the earliest reference of these date back to Ancient China. Melting wax with consistently spaced markings indicates the flow of time.

Hourglass  its principle of operation is very similar to that of the water clock, but instead of flowing water there is sand.

Pendulum clock  Christiaan Huygens (known for many discoveries including centrifugal force) built it based on the ideas and calculations of Galileo Galilei (the 16^{th}17^{th} century polymath, pioneer at building telescopes, father of modern physics, and the guy who dropped balls from the Leaning Tower of Pisa, and didn't drop the ball).

Pocket watch  developed in 16^{th} century, it used a balance spring instead of a pendulum and was the most popular kind of watch before the invention of wristwatches.

Wristwatch  initially worn mainly by women, later it was used in the army (pulling out a watch from your pocket is not the most convenient thing to do in situations where you shoot and can be shot). They became popular after World War I, as people's perception changed to "wristwatches are a manly thing".

Quartz watch  instead of a pendulum or a balance spring, it uses a tiny quartz crystal, which oscillates with a specific frequency when the electric current from a battery passes through it.

Atomic clock  uses the oscillations of a cesium 133 atom. The international standard definition of a second is the duration of 9,192,631,770 periods of radiation produced by the transition between two levels of the cesium 133 atom. Which in practice means... super accurate clocks.