Days in Month Calculator
Find how many days are in any month of any year including February in leap years. Enter values for instant results with step-by-step formulas.
Calculator
Adjust values & calculateAll Months of 2026
Formula
Each month has a fixed number of days except February, which has 28 in common years and 29 in leap years. A leap year occurs when the year is divisible by 4 but not by 100, unless also divisible by 400. The calculator also computes weekday and weekend counts based on the starting day of the month.
Last reviewed: December 2025
Worked Examples
Example 1: February in a Leap Year
Example 2: Planning for a 31-Day Month
Background & Theory
The Days in Month Calculator applies the following established principles and formulas. Date and time calculations underpin a vast range of applications from financial settlement to scheduling and age verification. The complexity arises because civil timekeeping uses irregular units: months have 28, 29, 30, or 31 days; years have 365 or 366 days; hours, minutes, and seconds use base-60 arithmetic; and time zones introduce offsets ranging from -12:00 to +14:00 relative to UTC. The Gregorian calendar's leap year rule is a compound condition: a year is a leap year if it is divisible by 4, except for century years, which must be divisible by 400. Thus 1900 was not a leap year but 2000 was. This rule keeps the calendar synchronized with the solar year to within about 26 seconds per year. For algorithmic date calculations, the Julian Day Number provides a continuous integer count of days since January 1, 4713 BCE, eliminating the irregularity of calendar months and making interval arithmetic straightforward. The Unix epoch, by contrast, counts seconds since 00:00:00 UTC on January 1, 1970, and is the basis of POSIX time used in most computing systems. ISO 8601 standardizes date and time representation as YYYY-MM-DD and combined datetime as YYYY-MM-DDTHH:MM:SS±HH:MM, ensuring unambiguous machine-readable interchange across locales that would otherwise differ in day/month/year ordering. Business day calculation requires excluding weekends and, optionally, a jurisdiction-specific list of public holidays. Duration calculations expressed in years, months, and days must account for the variable length of months, making them non-commutative: the interval from January 31 to February 28 is different from the interval from February 28 to March 31. Age calculation algorithms must handle the edge case of birthdays on February 29 and ensure that a person born on December 31 is not counted as one year older on January 1 of the following year until the clock passes midnight. Zeller's Congruence provides a closed-form formula to determine the day of the week for any Gregorian or Julian calendar date using only integer arithmetic.
History
The history behind the Days in Month Calculator traces back through the following developments. The need to track time and predict astronomical events gave rise to calendrical systems independently across many civilizations. The Babylonians, around 2000 BCE, developed a lunisolar calendar with 12 months of alternating 29 and 30 days, inserting an intercalary month periodically to keep pace with the solar year. They also divided the day into 24 hours and the hour into 60 minutes, a sexagesimal convention that persists in every modern clock. The Egyptian civil calendar used 12 months of exactly 30 days plus five epagomenal days, totaling 365 days. Though simple for administrative purposes, it drifted against the solar year by one day every four years. Julius Caesar, advised by the Egyptian astronomer Sosigenes, reformed the Roman calendar in 45 BCE. The Julian calendar introduced a 365-day year with a leap day every four years, a system that served Europe for over sixteen centuries. By the 16th century, the accumulated error of the Julian calendar had shifted the spring equinox ten days from its ecclesiastically mandated date, disrupting the calculation of Easter. Pope Gregory XIII commissioned the calendar reform that bears his name, and the Gregorian calendar was introduced in Catholic countries in October 1582. The transition required skipping ten days: October 4 was followed by October 15. Protestant and Orthodox countries adopted the reform slowly; Britain and its colonies switched in 1752, Russia not until 1918, and Greece in 1923. The expansion of railways in the 1840s created an urgent practical problem: each city operated on its own local solar time, making train timetables impossible to coordinate. British railways adopted Greenwich Mean Time as a standard in 1847. The International Meridian Conference of 1884 in Washington formalized the prime meridian at Greenwich and established the global framework of 24 time zones. Daylight saving time was first adopted nationally during World War I to reduce coal consumption. The development of atomic clocks after World War II led to the definition of Coordinated Universal Time (UTC) in 1960, accurate to nanoseconds. The Y2K problem of 1999-2000 demonstrated that two-digit year storage in legacy systems could cause widespread failures, prompting a global remediation effort costing an estimated 300 to 600 billion dollars.
Frequently Asked Questions
Formula
Days = Calendar lookup with leap year check: (Y%4=0 AND Y%100!=0) OR Y%400=0
Each month has a fixed number of days except February, which has 28 in common years and 29 in leap years. A leap year occurs when the year is divisible by 4 but not by 100, unless also divisible by 400. The calculator also computes weekday and weekend counts based on the starting day of the month.
Worked Examples
Example 1: February in a Leap Year
Problem: How many days are in February 2024?
Solution: Check leap year: 2024 / 4 = 506 (divisible by 4). 2024 is not a century year, so no further checks needed. 2024 IS a leap year. Therefore February 2024 has 29 days.\nWeekdays: February 1 is Thursday. 29 days = 4 full weeks + 1 day. Weekdays: 21, Weekend days: 8.
Result: February 2024 has 29 days (leap year) | 21 weekdays, 8 weekend days
Example 2: Planning for a 31-Day Month
Problem: How many weekdays are in October 2026?
Solution: October always has 31 days. October 1, 2026 is a Thursday.\nWeek 1: Thu-Fri (2 weekdays) + Sat-Sun (2 weekend)\nWeeks 2-5: 4 full weeks = 20 weekdays + 8 weekend\nRemaining: Oct 30 Fri (1 weekday), Oct 31 Sat (1 weekend)\nTotal weekdays: 2 + 20 + 1 = 22. Weekend days: 2 + 8 + 1 = 9.
Result: October 2026 has 31 days | 22 weekdays, 9 weekend days
Frequently Asked Questions
How many days are in each month of the year?
The twelve months of the Gregorian calendar have the following day counts: January has 31 days, February has 28 (or 29 in a leap year), March has 31, April has 30, May has 31, June has 30, July has 31, August has 31, September has 30, October has 31, November has 30, and December has 31. A common mnemonic is the knuckle method: make a fist and count across your knuckles and the valleys between them starting from January. Knuckles represent 31-day months, valleys represent 30-day months (except February). Seven months have 31 days, four months have 30 days, and February has either 28 or 29 days.
Why does February have fewer days than other months?
February has fewer days due to the ancient Roman calendar system. The original Roman calendar created by Romulus had only 10 months starting in March, and the winter period was not assigned to any month. King Numa Pompilius added January and February around 713 BC, but the year only had 355 days. February was the last month added and received the fewest days. When Julius Caesar reformed the calendar in 46 BC to create the Julian calendar with 365 days, he could have distributed the extra 10 days evenly, but he chose to add them to various months while leaving February short. Augustus Caesar later took a day from February to add to August, his namesake month, leaving February with just 28 days in common years.
How can I remember how many days are in each month?
There are several popular mnemonics for remembering month lengths. The knuckle method is the most reliable: hold your fists together and count across the knuckles and valleys, starting from your left pinky knuckle as January. Every knuckle represents a 31-day month, every valley between knuckles represents a 30-day month (except February). The traditional rhyme works too: Thirty days hath September, April, June, and November. All the rest have thirty-one, except February alone, which has twenty-eight days clear, and twenty-nine in each leap year. A simpler modern approach is to remember that months alternate between 31 and 30 days with two exceptions: February is short, and July-August are consecutive 31-day months.
Why do some months have 31 days and others have 30?
The uneven distribution of days across months is a historical artifact of Roman calendar politics rather than any astronomical necessity. The original Julian calendar alternated between 31 and 30 days, with February getting 29 (30 in leap years). However, when the Roman Senate named the eighth month after Augustus Caesar, they took a day from February and added it to August so that Augustus would not have fewer days than July (named after Julius Caesar). They then adjusted September through December to maintain a reasonable distribution, breaking the clean alternating pattern. This is why July and August are consecutive months with 31 days each, and why September through December alternate in the opposite pattern from what you might expect.
How do different calendar systems handle month lengths?
Different calendar systems take remarkably different approaches to organizing months. The Islamic (Hijri) calendar uses 12 lunar months alternating between 29 and 30 days, making each year about 354 days. The Hebrew calendar uses a similar lunar approach but adds a 13th month seven times in every 19-year cycle to stay aligned with the solar year. The Persian (Solar Hijri) calendar has six consecutive 31-day months, five consecutive 30-day months, and one month with 29 or 30 days. The Chinese calendar is lunisolar with months beginning at each new moon, alternating between 29 and 30 days. The Ethiopian calendar has 12 months of 30 days each plus a 13th month of 5 or 6 days.
What is the significance of knowing days in a month for business?
Knowing the exact number of days in a month is crucial for many business calculations. Payroll processing requires accurate day counts for prorated salaries, daily pay rates, and overtime calculations. Financial institutions use day counts for interest calculations, with different conventions like Actual/360 and Actual/365 producing different results. Rental and lease agreements often calculate monthly rates based on actual days, meaning February rent might differ from March rent on a daily-rate basis. Project management depends on accurate business day calculations for scheduling and deadline setting. Subscription services and SaaS companies use month lengths to calculate prorated charges when customers sign up or cancel mid-month.
References
Reviewed by Abdullah, Technical Content Specialist · Editorial policy