How to calculate month start and month end dates in Python

When creating business reports you'll often need to be able to calculate the month start and month end dates. Here's how to do it in Python.

How to calculate month start and month end dates in Python
Picture by Towfiqu Barbhuiya, Unsplash.
3 minutes to read

When creating business reports or running queries against a database or web analytics platform in a business setting, you’ll often need to know the start and end dates of the month.

In this quick project, we’ll create a couple of functions that will help us calculate the start and end dates of the month, so you can pass the values to your queries to extract the correct data.

Import the packages

First, open a Jupyter notebook and import the packages we’ll need. We can use three modules from the Python standard datetime library for this: date, datetime, and timedelta.

from datetime import date, datetime, timedelta

Calculate the date at the start of the month

Next, we’ll create a function to return the start date of the month in the common YYYY-MM-DD date format used in databases such as MySQL. To do this, we’ll pass in the year and month as integers, and then return a datetime object by setting the day to 1.

We’ll then convert the datetime object to a string in the format YYYY-MM-DD using strftime().

def get_first_date_of_current_month(year, month):
    """Return the first date of the month.

    Args:
        year (int): Year
        month (int): Month

    Returns:
        date (datetime): First date of the current month
    """
    first_date = datetime(year, month, 1)
    return first_date.strftime("%Y-%m-%d")

Now we can run the get_first_date_of_current_month() function to get the first date of the current month, simply by passing in the year and month.

first = get_first_date_of_current_month(2022, 1)
first
'2022-01-01'
first = get_first_date_of_current_month(2025, 5)
first
'2025-05-01'

Calculate the date at the end of the month

Calculating the month end date is a bit harder, since unlike the first day of the month, which is always a 1, the last date on the month could be a 31, 30, 28, or 29 depending on the month or year.

There are a few ways to do this, but I’ve gone with the following: last_date = datetime(year, month + 1, 1) + timedelta(days=-1).

This will calculate the first date of the following month, then subtract 1 day from it to get the last date of the current month. The timedelta module is excellent for calculating the difference between dates.

def get_last_date_of_month(year, month):
    """Return the last date of the month.
    
    Args:
        year (int): Year, i.e. 2022
        month (int): Month, i.e. 1 for January

    Returns:
        date (datetime): Last date of the current month
    """
    last_date = datetime(year, month + 1, 1) + timedelta(days=-1)
    return last_date.strftime("%Y-%m-%d")
last = get_last_date_of_month(2022, 1)
last
'2022-01-31'
last = get_last_date_of_month(2022, 4)
last
'2022-04-30'

Matt Clarke, Friday, March 18, 2022

Matt Clarke Matt is an Ecommerce and Marketing Director who uses data science to help in his work. Matt has a Master's degree in Internet Retailing (plus two other Master's degrees in different fields) and specialises in the technical side of ecommerce and marketing.