Something strange with date calculations

[Cathy20156688]

It seems like Excel doesn't count the 31st December as the 365th day in a common year. Instead it is using the number of day in a leap year (366 days) to caculate its dates.
When I was transforming the number of days, I realise that the result is one day off.
If 1 is 1st Jan 1900, then 365 should be 31st Dec 1900, since 1900 has 365 days.

[#2]

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If 1 is 1st Jan 1900, then 365 should be 31st Dec 1900, since 1900 has 365 days.

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Not in Excel. In Excel the year 1900 is a leap year.

[#3]

@Cathy20156688 

In addition, Excel incorrectly assumes that the year 1900 is a leap year - Office | Microsoft Docs