Date Calculation Tools

If you need to find the difference between two dates, the days between dates calculator gives you an exact count. For a broader breakdown that includes weeks and remaining days, use the weeks between dates calculator. The day counter adds the option to exclude weekends and holidays, making it ideal for business day calculations and project timelines.

For adding or subtracting time from a specific date, the date calculator lets you move forward or backward by days, weeks, months, or years. The days from now calculator and days ago calculator are simpler alternatives when your starting point is today. Use the age calculator when you need a complete breakdown of the time elapsed since a birthdate or any past event.

To identify what day of the week a date falls on, both the day of the week calculator and what day calculator serve this purpose. The day of year calculator tells you the ordinal day number (1 through 365 or 366), which is useful for Julian date references and year-progress tracking. The week number calculator provides ISO week numbers, commonly used in European scheduling, logistics, and project management.

Date calculations are rooted in the Gregorian calendar, the international civil calendar used by most countries. Introduced by Pope Gregory XIII in 1582, the Gregorian calendar corrects the drift of the older Julian calendar by refining the leap year rule: a year is a leap year if it is divisible by 4, except for years divisible by 100, which must also be divisible by 400 to qualify. This means 2000 was a leap year, but 1900 was not.

The ISO 8601 standard defines how dates and weeks are represented internationally. Under ISO 8601, weeks start on Monday, and the first week of the year is the one containing the first Thursday. This can cause the first few days of January to belong to the last week of the previous year, or the last days of December to belong to week 1 of the next year. Understanding this convention is important when working with week numbers in international business and software systems.

Julian Day Numbers (JDN) provide a continuous count of days since January 1, 4713 BC in the Julian proleptic calendar. Astronomers and historians use Julian Day Numbers to calculate intervals between events across different calendar systems without worrying about month lengths, leap years, or calendar reforms. Many date calculation algorithms convert dates to Julian Day Numbers internally, perform arithmetic, and then convert back.

Business day calculations exclude weekends (Saturday and Sunday in most Western countries) and optionally public holidays. Accurately counting business days requires knowing the holiday calendar for the relevant jurisdiction, as holidays vary widely between countries and even between states or provinces within the same country. Project managers, legal professionals, and payroll departments rely on business day calculations daily.