The Extra Day Hidden in Excel’s Calendar
Excel stores many dates as serial numbers: a date is represented by how many days it falls after a starting point. That makes calculations such as subtracting one date from another straightforward. But in Excel’s widely used 1900 date system, serial number 60 stands for February 29, 1900—a day that never existed.
The calendar rule is more precise than “every fourth year is a leap year.” A century year must also be divisible by 400 to qualify. Since 1900 is not, it had only 28 days in February. Excel’s date sequence nevertheless includes the imaginary day: in that system, March 1, 1900, comes after the phantom February 29.
Why Keep an Error?
The quirk came from early spreadsheet software and was retained by Excel for compatibility. If Excel removed the extra day, dates after February 1900 in existing workbooks would shift by one when interpreted under the corrected sequence. That could change date calculations in old files, even though most modern spreadsheets work with dates far removed from 1900.
So Excel preserves a historical mistake as part of its date system. It matters most when software handles dates around February and March 1900; for ordinary later dates, the calendar looks normal. The oddity is a reminder that a spreadsheet’s idea of a date is not just a calendar entry—it is also a numbered convention that old files depend on.
