Basic Formulas of Calendar
Here are some of the most important formulas for solving calendar reasoning problems:
S.No  To Calculate  Formulas 

Total Days in a Year 
Nonleap year: 365 days, Leap year: 366 days 

Total Weeks in a Year 
52 weeks (nonleap year) or 52 weeks + 1 day (leap year) 

Total Days in a Month 
Varies from 28 to 31 days depending on the month 

Number of Leap Years 
(Year / 4)  (Year / 100) + (Year / 400) 

Day of the Week 
Various algorithms like Zeller's Congruence, Doomsday Algorithm, etc., can be used to determine the day of the week for a given date. 

Number of Days between Two Dates 
Count the number of days in each year, month, and remaining days to calculate the total duration between two dates. 

Finding the Day on a Given Date 
Use the formula: Day = (Reference Day + Number of Days) % 7, where Reference Day is the known day and Number of Days is the duration to be added. 

The concept of odd days is very important in determining the days of the week. Let us look at how many odd days there will be in a century – i.e. 100 years. 
There will be 24 leap years and 76 non–leap years. As studied earlier each leap year and 2 odd days and each nonleap year has 1 odd day. Therefore, there will be 24 × 2 + 76 × 1 = 124 total odd days. Since 7 odd days make a week, to find out the net odd days, divide 124 by 7. The remainder is 5 – this is the number of odd days in a century. 

100 years 
5 odd days 

200 years 
5 x 2 = 10 – 7 (one week) 3 odd days. 

300 years 
5 x 3 = 15 – 14 (two weeks) 1 odd day. 

400 years 
{5 x 4 + 1 (leap century)} – 21} (three weeks) 0 odd days. 

Month of January 
31 – 28 = 3 odd days. 

Month of February 
28 – 28 = 0 odd day in a normal year and 1 odd day in a leap year and so on for all the other months. 
Quick Tip: Do you know? You can learn calendar formulas quickly if you first understand the fundamental concepts of calendar.
FAQsFAQs
How do you benefit from learning calendar formulas?
Learning calendar formulas provides a more profound understanding of the system we use to organize our time. These formulas help you to calculate dates, determine the day of the week for a particular date, find out leap years, and more.
This understanding is beneficial in various professional contexts, such as project management, event planning, and scheduling.
How to remember calendar formulas for a longer time?
Here are some strategies to help remember calendar formulas for a longer period:
Regular Practice: Regularly practice with calendar related questions and exercises to become more comfortable with the application of these formulas.
Understand the Concept: Dive into the fundamental principles of calendar concepts. By understanding the logic and reason behind each formula, you'll be able to remember and use them more effectively.
Use Visual Aids: Diagrams, flowcharts, or using a physical or digital calendar can be useful tools to visualize the calendar formulas, helping to reinforce your memory.
Link to RealLife Applications: Connect the formulas to reallife calendar applications. Understanding practical usage not only helps you remember the formulas, but also reveals their significance and practicality.
How conceptual understanding of the calendar topic helps in remembering its formulas?
A conceptual understanding of the calendar topic helps in remembering its formulas by offering an understanding of why these formulas work.
For example, understanding why a leap year occurs every four years helps you comprehend why we add an extra day to the calendar.
By connecting these formulas to the core calendar concepts, you create a stronger, more meaningful mental link that aids memory and application.