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# Why Is Serial Number 1 Dated December 31, 1899?
- URL: https://www.vizney.com/why-is-serial-number-1-dated-december-31-1899/
- Published: 2025-07-11T05:26:45.000Z
- Updated: 2025-07-11T05:26:45.000Z
- Author: Yehwan Hwang
- Tags: Knowledge Base

When converting a date to a number in Excel, Google Sheets or Tableau, you'll notice that the value `1` represents a specific point in time. In Excel, that number corresponds to **January 1, 1900**, but in Google Sheets and Tableau, it points to **December 31, 1899**. Why do the same numbers return different dates?

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### Dates are stored as numbers

Spreadsheets convert dates into **serial numbers**, which represent the number of days since a starting point. This allows for easy date calculations such as filtering, sorting, and time intervals.

Here’s how it works:

- Excel: `1 = 1900-01-01`, `2 = 1900-01-02`
- Google Sheets: `1 = 1899-12-31`, `2 = 1900-01-01`

The discrepancy lies in how each system handles a non-existent date: **February 29, 1900**.

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### Excel: Carrying forward a legacy bug

Excel treats **1900 as a leap year**, even though it wasn’t. This was done intentionally to maintain compatibility with an older spreadsheet program, **Lotus 1-2-3**, which made the same mistake.

As a result, Excel includes **February 29, 1900**, even though it never existed. This adds an extra day to the calendar, and shifts all serial numbers before March 1, 1900 by one.

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### Google Sheets: Based on accurate calendar logic

Google Sheets corrects this historical inaccuracy. It does **not** treat 1900 as a leap year and follows the actual **Gregorian calendar**. Therefore:

- `60 = 1900-02-28`
- `61 = 1900-03-01`

Because it doesn’t include the phantom February 29, 1900, the date system starts one day earlier: **December 31, 1899**.

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| Serial Number | Excel Date                 | Google Sheets Date |
| ------------- | -------------------------- | ------------------ |
| 1             | 1900-01-01                 | 1899-12-31         |
| 59            | 1900-02-28                 | 1900-02-27         |
| 60            | 1900-02-29 *(nonexistent)* | 1900-02-28         |
| 61            | 1900-03-01                 | 1900-03-01         |

💡

From March 1, 1900 onward, Excel and Google Sheets return the same dates for the same serial numbers.