Date & Day Calculator

What is a Date Day Calculator?

A Date Day Calculator is a web-based tool that helps you identify the day of the week for any specific date and calculate various date-related information. You can enter past or future dates to instantly find out what day of the week it was or will be, which day of the year it is, whether it’s a leap year, and more.

Key Features

Date Input and Selection

You can directly enter a date using the date picker or click on your desired date in the built-in mini calendar. The “Today” button automatically fills in the current date, and the “Random Date” button generates a random date between 1900 and 2100.

Day of Week Display

The calculator displays the day of the week for your selected date from Sunday to Saturday. Weekends (Saturday and Sunday) are distinguished with different colors from weekdays for easy visual identification.

Day of Year Calculation

Starting from January 1st as day 1, it calculates which day of the year your selected date falls on. It displays whether it’s one of 366 days in a leap year or 365 days in a common year.

Week Number Calculation

Following the ISO 8601 standard, it calculates which week of the year the date falls in. The week containing January 1st is counted as week 1.

Days Remaining

It calculates and displays the number of days remaining from your selected date until the end of that year. This is useful when planning annual goals or checking deadlines.

Leap Year Determination

Following Gregorian calendar rules, it determines whether the year is a leap year or common year. The leap year rules are:

• Years divisible by 4 are leap years
• Years divisible by 100 are common years
• Years divisible by 400 are leap years

For example, 2000 is a leap year, but 2100 is a common year.

Date Format Conversion

The selected date is converted and provided in various formats:

• ISO 8601 Format: International standard date format (e.g., 2025-10-26)
• Unix Timestamp: Time in seconds elapsed since January 1, 1970, 00:00:00 UTC

Mini Calendar

A monthly calendar view is provided so you can see all dates of the month at a glance. You can navigate to previous/next months using arrow buttons, and today’s date and the selected date are displayed in different colors.

Copy Function

All calculation results include a button that allows you to copy each item to your clipboard. Clicking the copy button instantly saves that information to your clipboard, and a completion notification appears at the bottom of the screen.

Search History

The 12 most recent searched dates are automatically saved in your browser’s local storage. You can click on dates in your history to retrieve their information again, and use the clear all button to reset your history.

Use Cases

Checking Historical Events

You can confirm what day of the week a specific past date was to identify the day when historical events occurred.

Planning Anniversaries

Check in advance what day of the week future anniversaries or important dates will fall on to plan your schedule accordingly.

Project Management

Calculate the days remaining until project deadlines or confirm which week a specific date falls in to manage your schedule effectively.

Data Analysis

Use it when converting dates to Unix timestamps or standardizing them to ISO 8601 format during data analysis work.

Understanding Calendar Systems

Gregorian Calendar

This calculator operates based on the Gregorian calendar. The Gregorian calendar is a calendar system introduced by Pope Gregory XIII in 1582 and is currently the most widely used standard calendar worldwide. It was designed to correct the errors in the Julian calendar and maintain the vernal equinox, consisting of 365 days in common years and 366 days in leap years.

Origin of Weekday Names

Modern weekday names originated from ancient Roman and Germanic celestial bodies and mythology:

• Sunday: Day of the Sun
• Monday: Day of the Moon
• Tuesday: Day of Tiw (Norse god of war)
• Wednesday: Day of Odin (Norse supreme god)
• Thursday: Day of Thor (Norse god of thunder)
• Friday: Day of Freya (Norse goddess of love and beauty)
• Saturday: Day of Saturn (Roman god of agriculture)

How to Use

1. Select or directly enter your desired date in the date picker.
2. Alternatively, click on a date in the mini calendar to select it.
3. Click the Calculate button, or the calculation happens automatically when you enter a date.
4. Check the day of the week and various date information in the results section.
5. Any needed information can be saved to your clipboard using each item’s copy button.

Browser Compatibility

This calculator works properly in the following browsers:

• Chrome (latest version)
• Firefox (latest version)
• Safari (latest version)
• Edge (latest version)

It is fully supported on mobile browsers and optimized for various screen sizes with responsive design.

Privacy Protection

This calculator does not collect user personal information. Search history is only saved in the user’s browser local storage and is not transmitted to external servers. Users can delete their history at any time.

Technical Specifications

• Implementation Languages: HTML5, CSS3, JavaScript (ES6+)
• Design Style: Neobrutalism
• Responsive Design: Mobile, tablet, desktop support
• Data Storage: Browser local storage
• Calculation Method: Based on JavaScript Date object

Version Information

• First Release: March 20, 2025
• Current Version: October 26, 2025
• Major Updates:
  • Improved result card layout (highlighted day of week)
  • Enhanced text size readability
  • Added mini calendar feature
  • Enhanced copy functionality
  • Added random date generation feature
  • Added history feature

Frequently Asked Questions

Can dates before 1582 be calculated?

This calculator uses JavaScript’s Date object, so it can calculate dates before 1582. However, dates before the introduction of the Gregorian calendar may differ from the Julian calendar, so please use them for reference only.

Are the calculation results accurate?

This calculator uses JavaScript’s standard Date object for calculations, providing accurate results based on the Gregorian calendar. However, for legal or official documents, we recommend having them verified by an expert.

How long is history retained?

The 12 most recent searched dates are stored in your browser’s local storage. They are retained until you clear your browser cache or press the clear all button.

Can it be used offline?

This calculator is a pure client-side application, so once you load the page, it can be used without an internet connection. (However, ads may not be displayed.)

Disclaimer

This calculator is provided for reference purposes. For legal or official documents, please always have them verified by an expert. For historical dates (before 1582), there may be differences from actual historical records due to the differences between the Julian and Gregorian calendars.