Understanding Excel Date to Number Conversion

In Microsoft Excel, dates are stored as serial numbers representing the number of days since January 1, 1900. This system allows for easy date arithmetic and conversion.

Excel Date System

Excel internally saves dates as numbers. For example, the date 10/26/2013 corresponds to the number 41573. This number is calculated as the days elapsed from a base date.

Converting Dates in Excel

To view the underlying number of a date, change the cell format to General. This will display the serial number. If the date is stored as text, use the DATEVALUE function to convert it to a number.

Excel's Date System Errors

Excel incorrectly treats 1900 as a leap year, which affects dates before March 1, 1900. The actual base date is December 30, 1899, to account for this error.

Code Example

To understand the conversion, here is a Python function that mimics Excel's date-to-number calculation:

import datetime

def excel_date_number(date_str):
    # Parse the date string in format MM/DD/YYYY
    date_obj = datetime.datetime.strptime(date_str, "%m/%d/%Y").date()
    base_date = datetime.date(1899, 12, 30)
    days_diff = (date_obj - base_date).days
    # Adjust for Excel's error for dates before 1900-03-01
    if date_obj < datetime.date(1900, 3, 1):
        days_diff -= 1
    return days_diff

# Example usage
print(excel_date_number("10/26/2013"))  # Should output 41573

This code demonstrates the logic behind Excel's date conversion, helping users understand the formula.