Joram Mutenge
2025-06-29
If I asked you, “How many days are in each month of the calendar?” most people would be able to answer for a few months, but not all.
I, on the other hand, never had this problem. At the age of 10, I memorized a line from a TV ad that went:
“Thirty days hath September, April, June, and November. All the rest have thirty-one, except for February, which has twenty-eight.”
I’ve never forgotten that line–partly because I loved the ad, and partly because it turned out to be surprisingly useful.
But let’s say you’re not as fortunate as I was, and you’re unsure whether April has 30 or 31 days. That’s where Polars can help.
Suppose we have the following dataframe with Month and Date columns:
| Month | Date |
|---|---|
| str | date |
| "Jan" | 2024-01-13 |
| "Feb" | 2024-02-15 |
| "Mar" | 2024-03-17 |
| "Apr" | 2024-04-19 |
| "May" | 2024-05-21 |
We can use the month_end function to determine the last day of each month. This tells us exactly how many days that month has.
| Month | Date | Month_End |
|---|---|---|
| str | date | date |
| "Jan" | 2024-01-13 | 2024-01-31 |
| "Feb" | 2024-02-15 | 2024-02-29 |
| "Mar" | 2024-03-17 | 2024-03-31 |
| "Apr" | 2024-04-19 | 2024-04-30 |
| "May" | 2024-05-21 | 2024-05-31 |
Now you know that April has only 30 days, without needing to memorize a rhyme from a TV ad from my childhood. Incidentally, that line is actually from a nursery rhyme.
Even more tricks from my Polars course!