Date Calculator

A date calculator has two common uses. It can calculate the time between two dates in years, months, weeks and days or it can add or subtract a period of time from a given date to find out what date will result. After selecting the mode and entering the data, the results panel is immediately updated according to the values entered.

Using the date calculator tool:

Select the operation to perform above. For "Number of days between two dates", enter a start and end date to get the total number of days, weeks, months, and working days between them. For "Add or subtract time span from a date", enter a starting date and a time span to get the resulting date and day of week.

For each date, information such as the day of week, day of year, ISO week number, quarter and whether it is a leap year are also shown. This information is updated in real time in both modes based on the input of the start date.

Number of days between two dates:

Calculating the time interval between two dates may seem simple but there are different ways to do it. In a normal calculation, the end date is not included. For example, January 1st to January 31st is 30 days. This is because you are counting the number of nights, rather than including both endpoints. This rule is used when calculating age or how long something has been going on for.

When measuring the length of an event (such as a hotel stay, project or rental period), you usually want to include both endpoints. If you turn on "Include end date", one day is added so that for example a booking from Monday through Friday will show up as 5 days instead of 4.

The results can be presented in three ways and you can choose the one that is most suitable for you: total number of days, representation in years, months, weeks and days or separate display of the sum for each unit. The panel also shows the number of weekdays and weekend days within this period.

Add or subtract date ranges

To find future or past dates, first set a reference date and then enter any combination of years, months, weeks and days to add or subtract. This is often used for setting deadlines. For example, contracts may say "90 days from today" while post-op checkups might be "6 weeks after surgery". The tool will return the exact date and corresponding day of the week.

Years and months are added according to the calendar, which is especially important at the end of a month. If you add one month to 1/31, it will be the last day of February, which is either the 28th or the 29th in a leap year. The 2/31 does not exist. A week consists of seven days, and if you specify a number of days, the date on the calendar will move forward one day at a time.

Weekdays and holidays

Business days are Monday through Friday. When the "Calculate only business days" option is selected, Saturdays and Sundays are skipped when adding or subtracting. If you count five business days from a Monday, you will arrive at the following Monday, not Wednesday. In difference mode, the panel displays the number of business days and weekend days within the specified range.

While this tool excludes weekends, it does not account for holidays as these vary by country and employer. If an accurate calculation of business days is required, you may need to make manual adjustments by subtracting any holidays within the date range that fall on a business day. Below is a list of US federal holidays for the next two years for reference.

Calculating months, weeks and years:

The year-month-day breakdown is done by successively subtracting complete units. First a full year is subtracted, then a full month and finally a full week, leaving the remaining days. Because each stage is only completed when the actual calendar day is reached, different month lengths and leap years are taken into account so that the sum of all parts always leads to the correct end date.

This is different from an estimate using fixed periods of time where each month is considered to have 30 days and each year is considered to have 365. Both approaches are useful. The calendar split is good for answering questions like "How old am I?" or "How far apart in time are these two dates?" while a simple count of days is good for calculating interest, notice periods, and transit times.

A brief history of the Gregorian calendar.

The current calendar is the Gregorian Calendar. It was improved from the Julian Calendar which was introduced by Julius Caesar in 46 BCE and set a year to be 365 days with an extra day added every four years. This rule was fairly accurate but not perfect. The Julian Year was about 11 minutes longer than a solar year, so by 1582 the calendar had already drifted ten days from the seasons.

Pope Gregory XIII corrected this discrepancy in the year 1582 by skipping ten days. He made October 15 the day after October 4. Also he tightened up the rules for leap years. Under the Gregorian calendar, a year is a leap year if it is divisible by 4 but century years are only leap years if they are also divisible by 400. So the year 2000 was a leap year while the years 1900 and 2100 will not be leap years. This change reduced the error from one day every 128 years to one day every 3,030 years. For this reason, more than four centuries later, this calendar still matches up with the seasons.

Note:

If possible start with the earlier date. Even if you enter dates in reverse order, the tool will work correctly and give a duration as a positive number. Use the "Include end date" switch when calculating the length of an event, and disable it when calculating age or time period. Note that for deadlines on working days, you must account for holidays yourself.

This tool is for general planning and educational purposes only. Please verify results with the appropriate authorities when it comes to legal deadlines, contracts or medical appointments as they may be calculated according to rules that start on day one, do not take into account holidays, or roll over to the next business day.