The Calendar
The calendar connects time measurement to math: a year is approximately 365¼ days (Earth's orbit around the sun), so we have common years with 365 days and leap years with 366. A decade is 10 years, a century is 100. Months vary from 28–31 days, with the rhyme 'Thirty days hath September...' helping students remember. Covered in Saxon Math Intermediate 4, this 4th grade math skill underpins scheduling, elapsed time, and understanding historical time spans.
Key Concepts
Property A year is the time Earth takes to orbit the sun, about $365\frac{1}{4}$ days. We have common years with 365 days and leap years with 366 days. A decade is ten years and a century is one hundred years. The jingle “Thirty days have September, April, June, and November” helps remember which months have 30 days.
Example How many days are in a leap year? A leap year has 366 days. How many days does December have? It's not in the “Thirty days” jingle, so it has 31 days. How many years are in 5 decades? $5 \times 10 = 50$ years.
Explanation Think of the calendar as our planet's personal diary! To keep the seasons in sync, we add an extra day every four years in a leap year to account for that pesky quarter day. This clever jingle helps you remember which months are shorter, so you're never caught off guard when making plans.
Common Questions
How many days are in a leap year?
A leap year has 366 days. An extra day, February 29, is added approximately every four years to account for the extra quarter-day Earth takes to orbit the sun each year.
How many months have 31 days?
Seven months have 31 days: January, March, May, July, August, October, and December. Using your knuckles to count (knuckle = 31 days, valley = 30 or fewer) is a classic memory trick.
What is a decade and a century?
A decade is 10 years and a century is 100 years. So 3 decades = 30 years and 2 centuries = 200 years.
Why do we have leap years?
Earth takes about 365.25 days to orbit the sun. Over four years, those extra quarter-days add up to one full day, so we add February 29 every four years to keep the calendar aligned with the seasons.
When do students learn about the calendar in math?
Calendar math — days, months, years, decades, centuries — is introduced in early grades and extended in 4th grade. Saxon Math Intermediate 4 covers leap years and conversions between calendar units.
How many days are in February in a non-leap year?
February has 28 days in a common (non-leap) year and 29 days in a leap year.