Fast calendar math,
without the clutter.

What is calendar math?

Calendar math is the everyday arithmetic of dates and time: counting how many days are between two dates, figuring out what date falls 90 days from today, working out how many business days a project actually takes, or converting a Unix timestamp into a readable date. It sounds simple, but month lengths, leap years, weekends, and time zones make it surprisingly easy to get wrong on paper. Calcadian exists to give you the right answer in a couple of seconds, every time.

Common date and time calculations

Most people land on a date calculator with one of a handful of questions. Calcadian's clutter-free date and time calculators cover the ones that come up most often:

• How many days between two dates? Plan trips, contracts, or anniversaries with the Date Difference Calculator.
• What date is X days from today? Add or subtract days, weeks, months, or years with the Add or Subtract Days Calculator.
• How many working days does this take? Skip weekends and custom holidays with the Business Days Calculator.
• How old am I exactly? Get years, months, and days from any date of birth with the Age Calculator.
• How long between two times? Track shifts and meetings with the Time Duration Calculator.
• Which ISO week is this date in? Use the Week Number Calculator for sprints and payroll.
• How many days until a deadline? Count down with the Days Until Calculator.
• What does this Unix timestamp mean? Convert epoch seconds with the Unix Timestamp Converter.
• How many weeks between two dates? Count weeks plus remaining days with the Weeks Between Dates Calculator.

Which Calcadian calculator should you use?

If you're not sure where to start, match your question to the closest tool:

• Counting any stretch of time between two dates → Date Difference.
• Projecting forward or backward from a known date → Add or Subtract Days.
• You care about workdays only → Business Days.
• You need a duration in years, months, and days → Age Calculator.
• You're measuring hours and minutes inside a single day → Time Duration.
• You're slotting a date into a week-numbered schedule → Week Number.
• You're counting down to a single fixed future date → Days Until.
• You're working with developer or log timestamps → Unix Timestamp Converter.
• You want weeks plus remaining days between two dates → Weeks Between Dates.

Calendar days vs business days

A calendar day is any day on the calendar — weekends and holidays included. A business day is a working day, typically Monday through Friday, excluding public holidays that you observe. The difference matters: a 10-calendar-day deadline can be 6, 7, or 8 business days depending on when it starts and which holidays fall inside it. Use the Date Difference Calculator for calendar days, and the Business Days Calculator when only working days count — it lets you add your own holiday list, since public holidays vary by country and even by region.

Dates, weeks, months, and time zones explained

Days, weeks, and months

Days are uniform: 24 hours each. Weeks are uniform too: 7 days. Months are not — they range from 28 to 31 days, which is why "3 months from today" and "90 days from today" almost never land on the same date. When precision matters, count in days. When you want a human-friendly answer, count in months and years.

ISO week numbers

ISO 8601 weeks start on Monday, and week 1 of any year is the week that contains the first Thursday. That means dates in early January or late December can belong to a different "ISO week year" than the calendar year — a quirk that catches a lot of people off guard around New Year.

Time zones and UTC

Calcadian's date calculators work in calendar-date terms, so time zones don't change the result. The Unix Timestamp Converter shows both your local time and UTC, since Unix time is always measured from the same UTC reference point. If you're coordinating across regions, UTC is the safe shared reference.

Privacy and performance

Every Calcadian tool runs in your browser. We don't send the dates you enter to a server, we don't require an account, and the pages stay lightweight so they load fast on phones and slow connections.