History of the Gregorian Calendar

The Gregorian calendar is the most prevalently used calendar today. Within this calendar, a standard year consists of 365 days with a leap day being introduced to the month of February during a leap year. The months of April, June, September, and November have 30 days, while the rest have 31 days except for February, which has 28 days in a standard year, and 29 in a leap year.

The Gregorian calendar is a reformed version of the Julian calendar, which was itself a modification of the ancient Roman calendar. The ancient Roman calendar was believed to be an observational lunar calendar, based on the cycles of the moon's phases. In 46 BC, the calendar was reformed by Julius Caesar, introducing an algorithm that removed the dependence of calendars from the observation of the new moon. In order to accomplish this, Caesar inserted an additional 10 days into the Republican calendar, making the total number of days in a year 365. He also added the intercalation of a leap day every fourth year.

Holidays and Date Differences

A holiday is a day that, either by custom or by law, is set aside such that regular activities like going to work or school are suspended, or at least reduced. Generally, holidays are meant to commemorate some event, person, or group of cultural or religious significance. In project management and planning, holidays and weekends are often excluded to determine true "working days".

Key Takeaways

• Leap Year Integrity. Calculations automatically compensate for leap years in the Gregorian calendar context.
• Dual Utility. Allows both measurement of duration between two known events, and the projection of a future/past date given an offset.