Greetings,
Not for tomorrow but I wanted to share some information I developed on Arne's request for a test suite for our functions in his comments of 5 July 2026.
Apparently OOXML doesn't specify a test suite either!
Nor do I think it is good practice for us to specify one. It's hard enough to say a definition correctly once, even harder to do it twice, over hundreds of functions.
But, in the interest of completeness, I did ask an AI about third party efforts at such testing. I don't warrant the following answer, only saying it was said:
*****AI starts here*****
Because the
Office Open XML (OOXML) standard treats spreadsheet formulas as unvalidated text strings, developers building calculation engines cannot rely on Microsoft or the ECMA/ISO standard bodies for validation suites. Instead, cross-compatibility and precision testing for financial and accounting functions (
PMT,
IRR,
XNPV, etc.) are heavily driven by
third-party open-source implementations, enterprise components, and headless engines.
[1, 2]The primary third-party projects and repositories used to validate accounting/financial functions against Excel/OOXML behaviors include:
1. Handsontable's HyperFormula Engine
- Project Details: HyperFormula is a headless, high-performance open-source spreadsheet calculation engine built in TypeScript. [1]
- Relevance to Accounting: It contains a complete implementation of roughly 400 popular spreadsheet functions, meticulously matching Excel's syntax and output outcomes. [1]
- Validation Value: Its extensive test framework offers some of the most reliable modern test vectors for validating complex financial math (such as cash flows, amortization, and multi-parameter interest functions) outside of a live Microsoft Excel runtime.
2. The LibreOffice "Calc" Core (ScInterpreter)
- Project Details: The Document Foundation's LibreOffice Calc source code houses the
ScInterpreter component. Calc acts as the de facto open-source baseline for OOXML and ODF compatibility. [1, 2] - Relevance to Accounting: Because financial operations require extreme precision (handling floating-point rounding disparities, specific day-count conventions like 30/360 or Actual/365, and payment-at-the-beginning/end variations), LibreOffice's unit test suites explicitly check formula evaluations against known Excel outcomes.
- Validation Value: Reviewing Calc's automated unit tests (
sc/qa/unit/) exposes thousands of rigorous reference tests designed to flag discrepancies between independent engine math and Microsoft Office behavior.
3. Open XML SDK (Proposed Formula Evaluation Feature)
- Project Details: The official .NET Open-XML-SDK has historically been limited to file structure manipulation, omitting calculation capabilities. However, community expansions have introduced native formula parsing and validation capabilities. [1, 2]
- Relevance to Accounting: The feature framework incorporates 650+ test cases encompassing various formula categories. [1]
- Validation Value: It explicitly tests edge cases, error conditions, and Excel-specific calculation bugs to ensure that independent .NET calculations output the exact string format expected within an OOXML
<v> (CellValue) schema node. [1, 2]
4. ClosedXML (ClosedParser Testing)
- Project Details: ClosedXML is a widely used .NET library that wraps the Open XML SDK to make file creation easier. They created ClosedParser, an independent high-performance formula engine.
- Relevance to Accounting: As noted by the maintainers, "OOXML documentation is notoriously inaccurate and not in-line with what Excel actually supports."
- Validation Value: The project uses automated data pipelines to extract formulas from thousands of real-world corporate spreadsheets, evaluating the calculated outputs against Excel's engine to prevent deviation in accounting formulas. [1]
*****/AI stops here*****