There is no 100th day in January, so DATE simple moves forward 100 days from January 1 and figures returns the correct date.
The formula on this page takes advantage of this behavior. The year assumed to be 2015 in this case, so 2015 is hard-coded for year, and 1 is used for month. The day value comes from column B, and the DATE function calculates the date as explained above.
Extracting a year value from a Julian date
If you have a date in a Julian format, for example, 10015, where the format is "dddyy", you can adapt the formula as follows:
The Excel DATE function creates a valid date from individual year, month, and day components. The DATE function is useful for assembling dates that need to change dynamically based on other values in a worksheet.
The Excel LEFT function extracts a given number of characters from the left side of a supplied text string. For example, LEFT("apple",3) returns "app".
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