Error with calculated date using DATE function

tlangford 

Could you please try to use semi colon instead of colon in your formula because based on your region of the site comma is not correct option

=DATE(YEAR([Due date])+4;MONTH([Due date]);DAY([Due date]))

See below from  official documentation 

Important

All example formulas in this topic use commas "," as the parameter delimiter character. In some countries, the comma is reserved for use as the decimal mark. In such countries, users creating a calculated field must use semi-colons ";" as the delimiter character. Regardless of which character is used when the field is created, the formula works on lists in SharePoint websites anywhere in the world. SharePoint automatically changes the delimiter character to the one that is appropriate for the language/culture of the current page. For example, suppose the following formula is created on a website whose culture setting is fr-fr (France): =IF(Number1>Number2;5;10). If the website's culture is then changed to en-us (United States), the formula changes automatically to: =IF(Number1>Number2,5,10).

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