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Date Handling

Codcel replicates Excel's date system, including its historical quirks. Understanding these details helps ensure your date calculations produce the expected results.

The Lotus 1-2-3 1900 Date Bug

Excel inherited a bug from Lotus 1-2-3 that treats 1900 as a leap year. In reality, 1900 was not a leap year (the next century-year that was a leap year is 2000). This means Excel's date serial number 60 corresponds to the non-existent date "29 February 1900".

How Codcel Handles This

By default, Codcel replicates this bug to ensure date serial numbers match Excel exactly. This is controlled by the Allow Lotus 1-2-3 1900 Date Bug setting (default: enabled).

  • Enabled (default): Date serial numbers match Excel exactly, including the phantom 29 Feb 1900
  • Disabled: Date serial numbers are astronomically correct, but may differ from Excel for dates before 1 March 1900

When to Disable

Only disable this setting if:

  • Your spreadsheet does not use date serial numbers
  • You need astronomically correct dates before 1 March 1900
  • You do not compare results against Excel

For most use cases, leave this setting enabled.

Date Serial Numbers

Excel uses serial numbers to represent dates:

Serial Number Date
1 1 January 1900
59 28 February 1900
60 29 February 1900 (does not exist)
61 1 March 1900
44927 1 January 2023

All dates from 1 March 1900 onward are off by one serial number compared to the astronomical calendar due to the bug above. Codcel matches Excel's numbering.

See Also