Date Calculator
Date arithmetic is more complex than it looks, months have 28, 30, or 31 days, leap years add an extra day every four years (with century-year exceptions), and business-day calculations must skip weekends. This calculator handles two tasks: finding the number of days, weeks, months, or years between two dates; and adding or subtracting a specific number of days to find a resulting date. Practical uses include payment due dates and late-payment penalties, project deadlines, contract notice periods, school term lengths, how many days until an event, and visa expiry dates. In Australian law, many deadlines (court filings, tax lodgements, notice periods) are measured in calendar days, while employment contracts often specify business days, this calculator shows both. Results include the day of the week for each date and the Julian day number for technical applications.
Frequently Asked Questions
How do I calculate days between two dates?
Subtract the earlier date from the later date, accounting for all month lengths and leap years. This calculator does it instantly and shows the result as total days, weeks, months, and years, plus business days excluding weekends. The calculation uses the standard Gregorian calendar algorithm to ensure accuracy across centuries of dates.
Does the calculator include or exclude the end date?
By default it counts the days between dates, not including the end date. So if you select January 1 to January 2, the result is 1 day. This matches the convention most people expect for 'how many days until' calculations. If you need to include the end date (counting 1 January to 2 January as 2 days), simply add 1 to the result.
How do I add or subtract days from a date?
To add or subtract days, enter your start date in the first field, then type the number of days to shift in the second field. Use a positive number to move forward in time (e.g., 14 to find a date 2 weeks from now) or a negative number to go backwards (e.g., -30 to find the date one month ago). The calculator automatically handles month boundaries, varying month lengths (28-31 days), and leap years. This is useful for working out payment due dates, project delivery timelines, shipping arrival estimates, or calculating how many days remain until an event.
Does this calculator handle leap years?
Yes, the calculator automatically handles all leap years according to the Gregorian calendar rule: any year divisible by 4 is a leap year, except century years (those ending in 00) which must be divisible by 400. This means 2000 was a leap year (divisible by 400), but 1900 and 2100 were and will be ordinary years. Without this rule, a simple day-counter would drift by about 1 day every 4 years, eventually becoming completely wrong. The calculator correctly includes February 29 in any calculation that spans a leap year.
What is the Julian day number?
The Julian day number is a count of consecutive days since noon UTC on January 1, 4713 BC (in the Julian calendar). It's used in astronomy to track time without calendar complexities, by some programming languages for date calculations, and by historians for ancient dates. The calculator shows the Julian day for both your start and end dates.
Can I work out what day of the week a date falls on?
Yes. The calculator displays the day of the week (Monday through Sunday) for your result date using a well-established calendar algorithm based on the Gregorian calendar system. This algorithm works for any date from 1583 onwards and correctly handles the full 400-year leap year cycle, meaning it will accurately identify weekdays for historical dates centuries in the past and future dates far ahead. This is useful for confirming what day a past event occurred, planning around a future date's weekday, or checking whether your birthday falls on a weekend this year.