Fix #2966: convert formula result to Date when numFmt is date-like

lib/stream/xlsx/worksheet-reader.js

@@ -297,6 +297,24 @@ class WorksheetReader extends EventEmitter {
                       } else {
                         cellValue.result = parseFloat(c.v.text);
                       }
+                      // Apply date-format conversion to formula results so
+                      // cells whose numFmt is date-like return a JS Date for
+                      // the result, mirroring the non-streaming reader.
+                      if (cellValue.result !== undefined && utils.isDateFmt(cell.numFmt)) {
+                        let numericResult = cellValue.result;
+                        if (typeof numericResult === 'string') {
+                          // Use Number() rather than parseFloat() so partial strings like
+                          // "2026-05-07" are rejected (Number returns NaN), not coerced to 2026.
+                          const parsed = Number(numericResult.trim());
+                          if (Number.isFinite(parsed)) {
+                            numericResult = parsed;
+                          }
+                        }
+                        if (typeof numericResult === 'number') {
+                          const date1904 = properties.model && properties.model.date1904;
+                          cellValue.result = utils.excelToDate(numericResult, date1904);
+                        }
+                      }
                     }
                     cell.value = cellValue;
                   } else if (c.v) {

lib/xlsx/xform/sheet/cell-xform.js

@@ -455,7 +455,22 @@ class CellXform extends BaseXform {
 
       case Enums.ValueType.Formula:
         if (model.result !== undefined && style && utils.isDateFmt(style.numFmt)) {
-          model.result = utils.excelToDate(model.result, options.date1904);
+          // Some writers serialize formula results with t="str" even when the
+          // result is a numeric date serial. Coerce string to number first so
+          // the date conversion runs. Non-numeric results (empty string, "N/A",
+          // booleans) are left unchanged.
+          let numericResult = model.result;
+          if (typeof numericResult === 'string') {
+            // Use Number() rather than parseFloat() so partial strings like
+            // "2026-05-07" are rejected (Number returns NaN), not coerced to 2026.
+            const parsed = Number(numericResult.trim());
+            if (Number.isFinite(parsed)) {
+              numericResult = parsed;
+            }
+          }
+          if (typeof numericResult === 'number') {
+            model.result = utils.excelToDate(numericResult, options.date1904);
+          }
         }
         if (model.shareType === 'shared') {
           if (model.ref) {

spec/integration/issues/issue-2966-formula-result-date-numfmt.spec.js

@@ -0,0 +1,126 @@
+'use strict';
+
+const fs = require('fs');
+const path = require('path');
+const JSZip = require('jszip');
+
+const ExcelJS = verquire('exceljs');
+
+// Programmatically build a workbook in memory whose sheet1.xml contains a
+// formula cell with the given inner XML. The cell uses numFmt "m/d/yyyy".
+async function buildWorkbookWithFormulaCell(cellInnerXml) {
+  const wb = new ExcelJS.Workbook();
+  const ws = wb.addWorksheet('Sheet1');
+  // Seed a placeholder formula+date cell so ExcelJS writes a valid styles.xml
+  // with the m/d/yyyy numFmt referenced as styleId 1.
+  const cell = ws.getCell('A1');
+  cell.value = {formula: 'TODAY()', result: new Date(2025, 0, 1)};
+  cell.numFmt = 'm/d/yyyy';
+
+  const buf = await wb.xlsx.writeBuffer();
+  const zip = await JSZip.loadAsync(buf);
+  const sheetPath = 'xl/worksheets/sheet1.xml';
+  let sheetXml = await zip.file(sheetPath).async('string');
+
+  // Replace the existing <c r="A1" ...>...</c> with a custom one that keeps
+  // the same styleId attribute (s="1") but uses the caller-supplied inner XML.
+  sheetXml = sheetXml.replace(/<c r="A1"([^>]*)>[\s\S]*?<\/c>/, (_match, attrs) => {
+    const styleAttr = (attrs.match(/\ss="\d+"/) || [' s="1"'])[0];
+    return `<c r="A1"${styleAttr}${cellInnerXml.attrs || ''}>${cellInnerXml.body}</c>`;
+  });
+
+  zip.file(sheetPath, sheetXml);
+  return zip.generateAsync({type: 'nodebuffer'});
+}
+
+describe('github issues', () => {
+  describe('issue 2966 - formula result with date numFmt', () => {
+    it('returns a JS Date when result is a numeric serial (numeric <v>, no t)', async () => {
+      const buf = await buildWorkbookWithFormulaCell({
+        attrs: '',
+        body: '<f>TODAY()</f><v>46143</v>',
+      });
+
+      const wb = new ExcelJS.Workbook();
+      await wb.xlsx.load(buf);
+      const cell = wb.getWorksheet('Sheet1').getCell('A1');
+
+      expect(cell.value).to.have.property('formula', 'TODAY()');
+      expect(cell.value.result).to.be.an.instanceOf(Date);
+      expect(Number.isNaN(cell.value.result.getTime())).to.equal(false);
+    });
+
+    it('returns a JS Date when result is a numeric serial marked as t="str"', async () => {
+      // Some writers serialize formula results as t="str" even when the
+      // cached value is a numeric date serial. The reader must still convert.
+      const buf = await buildWorkbookWithFormulaCell({
+        attrs: ' t="str"',
+        body: '<f>TODAY()</f><v>46143.20833333333</v>',
+      });
+
+      const wb = new ExcelJS.Workbook();
+      await wb.xlsx.load(buf);
+      const cell = wb.getWorksheet('Sheet1').getCell('A1');
+
+      expect(cell.value.result).to.be.an.instanceOf(Date);
+      expect(Number.isNaN(cell.value.result.getTime())).to.equal(false);
+    });
+
+    it('does not produce an Invalid Date when result is a non-numeric display string', async () => {
+      // Display-formatted strings cannot be reliably parsed as a numeric
+      // Excel serial; the reader must not fabricate an Invalid Date.
+      const buf = await buildWorkbookWithFormulaCell({
+        attrs: ' t="str"',
+        body: '<f>TODAY()</f><v>27/08/2025 19:33:34</v>',
+      });
+
+      const wb = new ExcelJS.Workbook();
+      await wb.xlsx.load(buf);
+      const cell = wb.getWorksheet('Sheet1').getCell('A1');
+
+      const {result} = cell.value;
+      // Must not be an Invalid Date.
+      if (result instanceof Date) {
+        expect(Number.isNaN(result.getTime())).to.equal(false);
+      } else {
+        expect(typeof result).to.equal('string');
+      }
+    });
+
+    it('streaming reader also returns a JS Date for numeric formula results', async () => {
+      const buf = await buildWorkbookWithFormulaCell({
+        attrs: ' t="str"',
+        body: '<f>TODAY()</f><v>46143.20833333333</v>',
+      });
+
+      const tmpPath = path.resolve(__dirname, '../../out', 'issue-2966-stream.xlsx');
+      fs.mkdirSync(path.dirname(tmpPath), {recursive: true});
+      fs.writeFileSync(tmpPath, buf);
+
+      const collected = [];
+      await new Promise((resolve, reject) => {
+        const reader = new ExcelJS.stream.xlsx.WorkbookReader(tmpPath, {
+          worksheets: 'emit',
+          styles: 'cache',
+          sharedStrings: 'cache',
+          hyperlinks: 'ignore',
+          entries: 'ignore',
+        });
+        reader.read();
+        reader.on('worksheet', worksheet => {
+          worksheet.on('row', row => {
+            collected.push(row.getCell(1).value);
+          });
+        });
+        reader.on('end', resolve);
+        reader.on('error', reject);
+      });
+
+      expect(collected).to.have.length(1);
+      const value = collected[0];
+      expect(value).to.have.property('formula', 'TODAY()');
+      expect(value.result).to.be.an.instanceOf(Date);
+      expect(Number.isNaN(value.result.getTime())).to.equal(false);
+    });
+  });
+});