Date Calculator

📊 Days in Each Month

📖 About Date Calculator

Date calculations appear in virtually every domain of life — legal deadlines, project planning, pregnancy tracking, financial settlements, visa expiry dates, and event countdowns. A proper date calculator must handle leap years, variable month lengths, and business day exclusions automatically and accurately. Getting even one day wrong on a legal notice or financial deadline can have serious consequences.

Our free date calculator supports three core functions: finding the exact number of days between two dates, adding days to a date to find a future date, and subtracting days to find a past date. It shows results in days, weeks, months, and years — with optional business day calculation that automatically excludes weekends. No signup required, works on all devices.

📜 History of Calendar Systems

The Gregorian calendar was introduced by Pope Gregory XIII in October 1582 to replace the Julian calendar, which had accumulated a drift of approximately 10 days over 1,300 years due to a slight miscalculation of the solar year. Catholic countries adopted it immediately, but Protestant and Orthodox countries resisted — Britain and its colonies did not switch until 1752, when they had to skip 11 days overnight (September 2 was followed by September 14). Russia did not adopt the Gregorian calendar until 1918, after the Revolution. Today, the Gregorian calendar is the international standard for civil purposes, though many cultures maintain their own traditional calendars alongside it for religious and cultural occasions.

📅 Common Date Calculation Scenarios

• Legal notices: 30-day response period starts from date of service
• Pregnancy: 280 days (40 weeks) from last menstrual period
• Visa validity: 90 days within any 180-day period (Schengen rule)
• Invoice terms: Net 30, Net 60, Net 90 payment deadlines
• Warranty periods: 1 or 2 years from purchase date
• Probation periods: 90 days from employment start date
• Financial settlements: T+2 (trade date plus 2 business days)

🎯 Common Uses

• Project deadlines — count working days to delivery date
• Age calculation — find exact age in days, weeks, months
• Event countdown — days until birthday, wedding, exam
• Legal deadlines — calculate 30 or 90 business day notices
• Visa and permit tracking — find expiry date from issue date
• Payroll and HR — calculate probation period end dates
• Historical research — find day of week for any past date

🧭 How to Use This Date Calculator

1. Select calculation mode — days between dates, add days, or subtract days
2. Enter your start date — use the calendar picker or type the date
3. Enter end date or number of days — depending on your mode
4. Toggle business days — exclude weekends if needed
5. Read results — days, weeks, months, years, and day of week

⚠️ Common Mistakes

• Off-by-one errors in day counting — "days between" two dates can be calculated inclusively or exclusively of the end date; always check which convention is needed.
• Ignoring calendar system differences — this calculator uses the Gregorian calendar; results will differ from Islamic (Hijri), Hebrew, or other calendar systems.
• Forgetting daylight saving shifts — for precise hour-level date/time math across a DST change date, the shift can add or subtract an hour.

🔗 Related Calculators

❓ How many days are in a year?

A standard year has 365 days. A leap year has 366 days, with February having 29 instead of 28 days. A year is a leap year if it is divisible by 4, except century years which must be divisible by 400 — so 2000 was a leap year but 2100 will not be. Our calculator automatically accounts for all leap years in every calculation.

❓ What is a business day?

A business day is any weekday that is not a public holiday — typically Monday through Friday. Saturdays and Sundays are excluded. Business days are used in legal notices, bank processing times, shipping estimates, and employment contracts. Our calculator can exclude weekends to give you the exact number of business days between any two dates.

❓ How do I calculate my age in days?

Enter your date of birth as the start date and today as the end date. The result shows your exact age in days, weeks, months, and years. For example, someone born on January 1, 2000 was exactly 9,132 days old on January 1, 2025. Most people are surprised how many days they have actually lived!

❓ What day of the week was I born?

Enter your date of birth in the start date field and click calculate. The result displays the exact day of the week for that date. For example, January 1, 2000 was a Saturday. This works for any date from 1900 to 2100 and is popular for personal curiosity, historical research, and verifying official documents.

❓ How many days until a specific date?

Enter today as the start date and your target date as the end date. The result shows the exact number of days remaining. This is useful for counting down to birthdays, weddings, exam dates, project deadlines, holidays, visa expiry dates, and contract end dates — any event you want to track precisely.

❓ How do I add 30 or 90 days to a date?

Select Add Days to a Date mode, enter your start date, and type 30 or 90 in the days field. The result shows the exact resulting date. This is commonly used for invoice payment terms (Net 30, Net 60, Net 90), visa and permit validity periods, warranty expiry calculations, and trial period end dates.

❓ What is the difference between calendar days and working days?

Calendar days count every day including weekends and holidays. Working days count only Monday to Friday, excluding weekends. Example: 10 calendar days starting on a Monday equals only 6 working days when weekends are excluded. Legal and financial deadlines always specify which applies — check carefully before calculating your deadline.

❓ How do I calculate a legal deadline date?

Enter your start date in the start field, select Add Days to a Date mode, enter the number of days required, and toggle business days if the deadline specifies working days. The result shows your exact deadline date and the day of the week it falls on. Always verify whether your specific legal context uses calendar days or business days, as this significantly affects the final date.