Function Problems

Why EOMONTH Needs a Double Unary with Excel Arrays

Steve K Oct 7, 2026 870 views

Question details

The user wants to understand why the EOMONTH function in Excel requires a double unary operator (--) when processing array values, and how to apply it to resolve array formula errors.

Why EOMONTH Needs a Double Unary with Excel Arrays

Product: Spreadsheets
Device & OS: not provided
Scenario: Using the EOMONTH function within an array formula that contains text-formatted dates or logical values, which causes calculation errors.
Observed behavior: The formula engine fails to process the array values automatically as numbers, requiring a double unary operator to coerce the text or logical data into numerical serial dates before EOMONTH can evaluate them.

Before you start

Ensure your array data contains valid date strings or logical values that can actually be converted into numeric date serial numbers, and verify if your spreadsheet software version supports dynamic arrays.

Solution 1Recommended

Apply the Double Unary Operator (--) to Coerce Array Values

Use a double negative operator to force the formula engine to convert text-formatted dates or logical values within an array into numeric serial numbers that the EOMONTH function can process.

The EOMONTH function strictly expects numeric inputs to represent dates. When you feed it an array generated from logical tests (TRUE/FALSE) or text strings, the formula engine might not automatically convert these non-numeric data types. The double unary operator (--) essentially multiplies the values by -1 twice, a mathematical operation that safely forces this conversion into numeric values without altering the underlying result.

Select the output cell

Click on the cell or range of cells where you want the end-of-month dates to be displayed.

Enter the EOMONTH formula with the operator

Type your EOMONTH formula, placing the double unary operator (--) immediately before the array argument. For example, enter =EOMONTH(--A2:A10, 0).

Evaluate the array formula

If you are using a legacy version of Excel, press Ctrl + Shift + Enter to evaluate the formula as an array. In modern software with dynamic arrays, simply press Enter.

Format as Date

Because EOMONTH returns serial numbers, highlight the resulting output cells, right-click, select 'Format Cells', and apply a 'Date' format to view the results correctly.

Troubleshooting Errors: If you receive a #NAME? error, check for spelling mistakes in the function name. If you receive a #VALUE! error, it means the array contains text that cannot be mathematically coerced into a date.

Solution 2

Update to Modern Dynamic Array Formulas

Ensure you are using a modern spreadsheet version that supports dynamic arrays natively, which handles many implicit coercions and spilling behaviors better than legacy array formulas.

Easily Handle Array Formulas with WPS Spreadsheets

WPS Spreadsheets provides robust support for advanced array formulas, modern dynamic arrays, and date functions like EOMONTH, ensuring seamless data coercion and high compatibility with your existing workbooks.

1. Open your file in WPS Spreadsheets: Launch WPS Office and open the workbook containing your date arrays.
2. Apply the double unary operator: Click on the target cell and type =EOMONTH(--[Your_Array_Range], [Months]).
3. Execute the formula: Press Enter to calculate. WPS Spreadsheets' advanced engine will automatically process the double unary coercion.
4. Format the results: Highlight the calculated cells, right-click, choose 'Format Cells', and select your preferred Date format.

Fully compatible with Microsoft Excel formulas and array operationsSupports modern dynamic arrays and advanced date functions like EOMONTHFree and lightweight alternative for heavy data processingFamiliar user interface requires zero learning curve

microsoft office alternative - wps office

Frequently Asked Questions

What exactly does the double unary operator (--) do in formulas?

The double unary operator consists of two minus signs. It performs a quick mathematical operation (multiplying by -1 twice) that converts text-represented numbers, dates, or boolean values (TRUE/FALSE) into actual numeric values (like 1/0 or date serial numbers) that spreadsheet functions can properly process.

Why am I getting a #VALUE! error with the EOMONTH function?

The #VALUE! error in EOMONTH typically occurs when the provided start_date argument is not recognized as a valid date or number. If you are passing an array of text-formatted dates, adding the double unary operator (--) before the array usually fixes this by forcing a numeric conversion.

Do I still need to use Ctrl+Shift+Enter for EOMONTH array formulas?

If you are using legacy spreadsheet software (like Excel 2019 or earlier), you must press Ctrl+Shift+Enter to evaluate array formulas. In modern software like Microsoft 365, Excel 2021, and recent versions of WPS Office, dynamic arrays process automatically when you simply press Enter.

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