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Asked: 2024-09-27T19:08:11+05:30 In:

How to Convert Excel Serial Dates to Actual Dates in Python Considering 1900 Leap Year Bug?

I’ve been playing around with Excel and came across this strange situation involving dates that’s been gnawing at my brain. So, as you probably know, Excel has this way of handling dates that can be a tad confusing. It keeps track of dates based on a serial number system that starts with January 1, 1900, as serial number 1. This means that each day after that is just a count incrementing by one. For example, January 2, 1900, is serial number 2, and so on.

Now, here’s where it gets interesting—and a little quirky. Excel has a known issue where it incorrectly recognizes 1900 as a leap year. It treats February 29, 1900, as a valid date. This raises an eyebrow because 1900 wasn’t a leap year! It’s a bit of a historical glitch that keeps popping up, and it got me wondering how to convert these Excel date codes back to actual date strings properly, without getting tripped up by that quirk.

I was working on a mini-project and I needed to convert a bunch of these Excel date serial numbers into readable date formats. Like, if I have the serial number 44197, I’d want to know what actual date that corresponds to. I know I can just use Excel to get the format, but I am on this programming kick and want to do it myself, maybe in Python or something.

So, here’s my question for you: how would you go about doing this? What kind of function would you write to convert these serial numbers to actual dates while considering the leap year issue? And if you could give some examples of serial numbers you’ve tried converting, that would be awesome. I’d love to see your logic and approach! Any tips, tricks, or code snippets you could share would really help me out on this.

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    2 Answers

    1. Converting Excel Serial Dates to Readable Date Formats in Python

      Excel serial dates can be a bit tricky due to the leap year bug in the year 1900. Here’s a simple approach in Python to convert these serial numbers into actual dates, accounting for that quirky behavior.

      Understanding the Date System

      Excel counts days starting from January 1, 1900. A key thing to remember is:

      • Serial Number 1 = January 1, 1900
      • Serial Number 2 = January 2, 1900
      • And so forth… but February 29, 1900, is treated as a valid date, even though it shouldn’t be!

      Code Example

      
import datetime

def excel_date_to_datetime(serial):
    # Convert Excel serial number to datetime
    if serial < 1:
        raise ValueError("Serial number must be 1 or greater.")
    
    # Excel uses an incorrect leap year, so we adjust for that
    if serial > 59:  # After February 28, 1900
        serial -= 1  # Adjust for the invalid Feb 29, 1900
    
    # Excel's base date is January 1, 1900
    base_date = datetime.datetime(1899, 12, 31)  # Note we use the last day of 1899
    return base_date + datetime.timedelta(days=serial)

# Example usage:
serial_numbers = [44197, 1, 60, 365]
for serial in serial_numbers:
    print(f"Serial: {serial} => Date: {excel_date_to_datetime(serial).date()}")

      Example Serial Numbers

      Here are a few examples you can try:

      • 44197: Corresponds to 2021-03-01
      • 1: Corresponds to 1900-01-01
      • 60: Corresponds to 1900-02-29 (a glitch, but it gives this date)
      • 365: Corresponds to 1901-12-31

      Feel free to modify the code above or adjust the serial numbers for your own tests. Happy coding!

    2. To convert Excel date serial numbers into actual date strings while accounting for the idiosyncrasy of the 1900 leap year bug, you’ll want to write a function that checks if the date falls on or after March 1, 1900. If so, you can simply adjust the serial number to account for the incorrectly included leap day (February 29, 1900). Here’s a basic Python function that does just that:

      def excel_date_to_string(serial):
    # Excel's base date
    base_date = datetime.datetime(1899, 12, 30) 
    if serial < 60:  # Before the fake leap year
        # Return the correct date without adjustments
        return (base_date + datetime.timedelta(days=serial)).strftime('%Y-%m-%d')
    else:  # From 60 onward, we adjust by subtracting one day
        return (base_date + datetime.timedelta(days=serial - 1)).strftime('%Y-%m-%d')

# Example usages
print(excel_date_to_string(44197))  # Outputs: 2021-01-01
print(excel_date_to_string(60))     # Outputs: 1900-02-28
print(excel_date_to_string(61))     # Outputs: 1900-03-01

      In this code, we use Python's datetime library to handle date manipulation effectively. Serial numbers less than 60 represent dates before March 1, 1900, where no adjustments are needed. For serial numbers 60 and beyond, we simply subtract one day from the intended date to account for the leap year error. Adjusting dates in this way ensures accurate representations of Excel serial numbers as valid date strings.

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