Calendar, Week & Day Math Calculator

Under international standard ISO-8601, weeks begin on Monday (Day 1) and end on Sunday (Day 7). Week 1 of any calendar year is officially defined as the week containing the first Thursday of that year (which is equivalent to the week containing January 4th). Because a week belongs to the year in which the majority of its four or more days lie, the initial days of January can occasionally belong to Week 52 or 53 of the preceding year, or late December days can belong to Week 1 of the upcoming year.

A calendar year in the modern Gregorian system is a leap year if it meets the following mathematical conditions:

• It is evenly divisible by 4;
• Except if it is divisible by 100, in which case it is not a leap year;
• Unless it is also divisible by 400, in which case it is a leap year.

This 400-year quadricentennial cycle accounts for Earth's actual solar orbital period of approximately 365.24219 days, maintaining astronomical equinox alignment within 1 day every 3,030 years.

In a standard 365-day common year, 365 ÷ 2 = 182.5 days. Exactly 182.5 days have elapsed at 12:00 PM (noon) on July 2nd (Day 182), leaving exactly 182.5 days remaining until midnight December 31st. In a 366-day leap year, 366 ÷ 2 = 183 days, which places the exact midpoint cleanly at 12:00 AM (midnight) on July 1st (Day 183).

Every single one of the 12 months in the calendar year has at least 28 days! While February famously has only 28 days (or 29 during leap years), the other eleven months have 30 or 31 days, meaning they also contain 28 full days plus 2 or 3 additional days.