diff --git a/src/io/excellOffice.test.ts b/src/io/excellOffice.test.ts
index 012e8d1..863b2e9 100644
--- a/src/io/excellOffice.test.ts
+++ b/src/io/excellOffice.test.ts
@@ -21,6 +21,7 @@ import {
   workbookPrintAreas,
   workbookRelationshipTargets,
   workbookSheetRefs,
+  workbookUsesDate1904,
   worksheetXmlToAutoFilter,
   worksheetXmlToCells,
   worksheetXmlToCellFormats,
@@ -615,6 +616,38 @@ describe("excellOffice OOXML helpers", () => {
     ).toBe("sheet-7");
   });
 
+  it("detects the XLSX 1904 date system flag from workbook properties", () => {
+    expect(workbookUsesDate1904({ workbook: { workbookPr: { date1904: "1" } } })).toBe(true);
+    expect(workbookUsesDate1904({ workbook: { workbookPr: { date1904: "true" } } })).toBe(true);
+    expect(workbookUsesDate1904({ workbook: { workbookPr: { date1904: "0" } } })).toBe(false);
+  });
+
+  it("converts imported XLSX 1904 date serials to the QExcell 1900 serial system", async () => {
+    const zip = new JSZip();
+    zip.file(
+      "xl/workbook.xml",
+      `<workbook><workbookPr date1904="1"/><sheets><sheet name="Dates" sheetId="1" r:id="rId1"/></sheets></workbook>`
+    );
+    zip.file(
+      "xl/_rels/workbook.xml.rels",
+      `<Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships"><Relationship Id="rId1" Target="worksheets/sheet1.xml"/></Relationships>`
+    );
+    zip.file("xl/styles.xml", `<styleSheet><cellXfs count="3"><xf numFmtId="0"/><xf numFmtId="14"/><xf numFmtId="20"/></cellXfs></styleSheet>`);
+    zip.file(
+      "xl/worksheets/sheet1.xml",
+      `<worksheet><sheetData><row r="1"><c r="A1" s="1"><v>43465</v></c><c r="B1"><v>5</v></c><c r="C1" s="2"><v>0.5</v></c></row></sheetData></worksheet>`
+    );
+
+    const blob = await zip.generateAsync({
+      type: "blob",
+      mimeType: "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
+    });
+    const imported = await importXlsxFile(new File([blob], "Date1904.xlsx", { type: blob.type }));
+
+    expect(imported.sheets[0].cells).toEqual({ A1: "44927", B1: "5", C1: "0.5" });
+    expect(imported.sheets[0].cellFormats).toEqual({ A1: { numberFormat: "m/d/yy" }, C1: { numberFormat: "h:mm" } });
+  });
+
   it("сохраняет и импортирует активный лист XLSX через workbookView activeTab", async () => {
     const blob = await exportXlsxBlob({
       id: "book-1",
diff --git a/src/io/excellOffice.ts b/src/io/excellOffice.ts
index 9814498..c306967 100644
--- a/src/io/excellOffice.ts
+++ b/src/io/excellOffice.ts
@@ -226,6 +226,7 @@ export async function importXlsxFile(file: File): Promise<ImportedExcellWorkbook
   const themeColors = await readOptionalWorkbookThemeColors(zip, parser);
   const styleTable = xlsxStylesXmlToCellFormatTable(await readOptionalWorkbookStyles(zip, parser), themeColors);
   const sheetRefs = workbookSheetRefs(workbookXml);
+  const date1904 = workbookUsesDate1904(workbookXml);
   const printAreas = workbookPrintAreas(workbookXml, sheetRefs);
   const relationshipTargets = workbookRelationshipTargets(relationshipsXml);
   const sheets = await Promise.all(
@@ -249,7 +250,7 @@ export async function importXlsxFile(file: File): Promise<ImportedExcellWorkbook
       const sheetRelationshipsXml = await readOptionalSheetRelationships(zip, parser, sheetPath);
       const commentsPath = worksheetCommentsPath(sheetRelationshipsXml, sheetPath);
       const commentsXml = commentsPath ? await readOptionalParsedXml(zip, parser, commentsPath) : undefined;
-      const parsedSheet = parsedWorksheetToSheet(sheetXml, sheet, index, sharedStrings, sheetRelationshipsXml, styleTable, themeColors, commentsXml);
+      const parsedSheet = parsedWorksheetToSheet(sheetXml, sheet, index, sharedStrings, sheetRelationshipsXml, styleTable, themeColors, commentsXml, date1904);
       const charts = await worksheetXmlToCharts(zip, parser, sheetXml, sheetRelationshipsXml, sheetPath, parsedSheet.name);
       return { ...parsedSheet, ...(printAreas[sheet.id] ? { printArea: printAreas[sheet.id] } : {}), charts };
     })
@@ -358,7 +359,8 @@ export function parsedWorksheetToSheet(
   relationshipsXml?: unknown,
   styleTable: Array<CellFormat | undefined> = [],
   themeColors: string[] = defaultXlsxThemeColors,
-  commentsXml?: unknown
+  commentsXml?: unknown,
+  date1904 = false
 ): ParsedSheet {
   const frozenPane = worksheetXmlToFrozenPane(worksheetXml);
   const autoFilter = worksheetXmlToAutoFilter(worksheetXml);
@@ -383,7 +385,7 @@ export function parsedWorksheetToSheet(
     ...(Object.keys(hiddenRows).length > 0 ? { hiddenRows } : {}),
     ...(showGridLines === false ? { showGridLines } : {}),
     ...(zoomScale ? { zoomScale } : {}),
-    cells: worksheetXmlToCells(worksheetXml, sharedStrings),
+    cells: worksheetXmlToCells(worksheetXml, sharedStrings, styleTable, date1904),
     mergedCells: worksheetXmlToMergedCells(worksheetXml),
     hyperlinks: worksheetXmlToHyperlinks(worksheetXml, relationshipsXml),
     comments: worksheetCommentsXmlToComments(commentsXml),
@@ -392,7 +394,7 @@ export function parsedWorksheetToSheet(
   };
 }
 
-export function worksheetXmlToCells(worksheetXml: unknown, sharedStrings: string[]) {
+export function worksheetXmlToCells(worksheetXml: unknown, sharedStrings: string[], styleTable: Array<CellFormat | undefined> = [], date1904 = false) {
   const cells: Record<string, string> = {};
   const sharedFormulas: SharedFormulaMap = {};
   const parsedCells = worksheetCellRecords(worksheetXml);
@@ -408,9 +410,10 @@ export function worksheetXmlToCells(worksheetXml: unknown, sharedStrings: string
 
   parsedCells.forEach(({ address, cell }) => {
     const text = xlsxCellToText(cell, sharedStrings, sharedFormulas, address);
+    const importedText = date1904 ? date1904AdjustedCellText(cell, text, styleTable[numberAttribute(cell.s)]) : text;
 
-    if (text !== "") {
-      cells[address] = text;
+    if (importedText !== "") {
+      cells[address] = importedText;
     }
   });
 
@@ -1032,6 +1035,12 @@ export function workbookActiveSheetId(workbookXml: unknown, sheets: Array<{ id:
   return sheets[activeTab]?.id ?? "";
 }
 
+export function workbookUsesDate1904(workbookXml: unknown) {
+  const workbook = childRecord(workbookXml, "workbook");
+  const workbookPr = childRecord(workbook, "workbookPr");
+  return booleanAttribute(workbookPr.date1904 ?? workbookPr["@_date1904"]);
+}
+
 export function workbookPrintAreas(workbookXml: unknown, sheets: Array<{ id: string; name: string }>): Record<string, string> {
   const workbook = childRecord(workbookXml, "workbook");
   const definedNames = childRecord(workbook, "definedNames");
@@ -1789,6 +1798,37 @@ function normalizedNumberFormat(value?: string) {
   return raw.slice(0, 255);
 }
 
+function date1904AdjustedCellText(cell: XmlRecord, text: string, format?: CellFormat) {
+  if (!isNumericXlsxCell(cell) || !isDateSerialXlsxNumberFormat(format?.numberFormat)) {
+    return text;
+  }
+
+  const serial = Number(text);
+  if (!Number.isFinite(serial) || serial < 0) {
+    return text;
+  }
+
+  return normalizedXlsxNumberText(serial + 1462);
+}
+
+function isNumericXlsxCell(cell: XmlRecord) {
+  const type = typeof cell.t === "string" ? cell.t.trim() : "";
+  return type === "" || type === "n";
+}
+
+function isDateSerialXlsxNumberFormat(value?: string) {
+  const pattern = normalizedNumberFormat(value)
+    .replace(/"[^"]*"/g, "")
+    .replace(/\\./g, "")
+    .replace(/\[[^\]]+]/g, "")
+    .toLowerCase();
+  return /[dy]/.test(pattern) || (pattern.includes("m") && !/[hs]/.test(pattern));
+}
+
+function normalizedXlsxNumberText(value: number) {
+  return Number.isInteger(value) ? String(value) : String(Number(value.toFixed(10)));
+}
+
 function xlsxNumberFormats(styleSheet: XmlRecord) {
   const formats = { ...builtinNumberFormats };
   toArray(childRecord(styleSheet, "numFmts").numFmt).forEach((numFmt) => {