ECMA-376 (XLSX) does not specify a default for code 30, hence nothing is in the SSFImplicit table. It further states:

When values not present in the lists below are used, the behavior is implementation-defined.

There are some locale-specific overrides for that particular code:

XLSB (2.5.76 Ifmt) refers to XLSX

XLS (2.4.126 Format) is even stranger, not prescribing a default for 30 and stating

The value of ifmt.ifmt MUST be a value within one of the following ranges or within 383 to 392.

• 5 to 8
• 23 to 26
• 41 to 44
• 63 to 66
• 164 to 382

Out of curiosity, can you repeat that process and save the file in a few different formats, namely "Excel 97-2003 Workbook (.xls)" and "SYLK (Symbolic Link) (.slk)"?

Out of curiosity, can you repeat that process and save the file in a few different formats, namely "Excel 97-2003 Workbook (.xls)" and "SYLK (Symbolic Link) (.slk)"?

For *.xls, SSF.is_date(SSF._table[fmtid] || String(fmtid)) will skip fmt 30 and read as normal number

For *.slk, the behavior is opposite to *.xls.
Custom date format can be recognized but normal date format cant.

ECMA-376 (XLSX) does not specify a default for code 30, hence nothing is in the SSFImplicit table. It further states:

When values not present in the lists below are used, the behavior is implementation-defined.

Certainly, ECMA-376 not includes code 30, number format to string should be implementation-defined.
I just considering whether to add these special cases when cellDates: true, these values should be converted to Date .

We'd have to test across a few more locales. If Excel is always treating format code 30 as the default date format, we can just add it to SSFImplicit.

Can you share the SLK file? You may need to compress as ZIP and attach.

😉Request for an extra help, I want to know which custom format code should be recognized as Date, is there any specs to read ?
Many thanks!

We'd have to test across a few more locales. If Excel is always treating format code 30 as the default date format, we can just add it to SSFImplicit.

Can you share the SLK file? You may need to compress as ZIP and attach.

sheetjs-bug.slk.zip

Thanks!

The function is a stripped down version of the tokenizer https://github.com/SheetJS/ssf/blob/master/bits/81_fmttype.js#L3

It looks for the following date tokens:

• B1 and B2 (calendar system)
• Y and E (year and era)
• M, D, H, S (month/minute, day, hour, second)
• A/P, AM/PM, 上午/下午 (12-hour indicator)

Can you check what version of the library you are using (console.log(XLSX.version)) ?
With 0.17.0 setting cellDates: true generates dates for every cell:

> require("xlsx").readFile("sheetjs-bug.xlsx", {cellNF: true, cellDates: true}).Sheets.Sheet1
...
  A2: { t: 'd', v: 2021-08-10T04:00:00.000Z, w: '8/10/21' },
  B2: { t: 'd', v: 2021-08-10T04:00:00.000Z, w: '8/10/21' },
  A3: { t: 'd', v: 2021-08-11T04:00:00.000Z, w: '8/11/21' },
  B3: { t: 'd', v: 2021-08-11T04:00:00.000Z, w: '8/11/21' },
  A4: { t: 'd', v: 2021-08-12T04:00:00.000Z, w: '8/12/21' },
  B4: { t: 'd', v: 2021-08-12T04:00:00.000Z, w: '8/12/21' },
  A5: { t: 'd', v: 2021-08-13T04:00:00.000Z, w: '8/13/21' },
  B5: { t: 'd', v: 2021-08-13T04:00:00.000Z, w: '8/13/21' },
...

With cellNF: true it looks like the format for cell B2 is missed (so something is still incorrect):

  A2: { t: 'n', v: 44418, z: 'm/d/yy', w: '8/10/21' },
  B2: { t: 'n', v: 44418, z: undefined, w: '8/10/21' },
  A3: { t: 'n', v: 44419, z: 'm/d/yy', w: '8/11/21' },

Can you check what version of the library you are using (console.log(XLSX.version)) ?

My options:

XLSX.read(ev.data.arrayBuffer, {
    type: 'array',
    cellFormula: false,
    cellHTML: false,
    cellText: false,
    cellStyles: true,
    cellDates: true,
    sheetRows: 100000,
    WTF: true,
}),

Addtional: I'm using xlsx in Worker.

Can you check what version of the library you are using (console.log(XLSX.version)) ?

My options:

XLSX.read(ev.data.arrayBuffer, {
    type: 'array',
    cellFormula: false,
    cellHTML: false,
    cellText: false,
    cellStyles: true,
    cellDates: true,
    sheetRows: 100000,
    WTF: true,
}),

Addtional: I'm using xlsx in Worker.

Ok finally i find cellText: false will cause this error.

cellText: true

// SSF._table
{
    "0": "General",
    "1": "0",
    "2": "0.00",
    "3": "#,##0",
    "4": "#,##0.00",
    "9": "0%",
    "10": "0.00%",
    "11": "0.00E 00",
    "12": "# ?/?",
    "13": "# ??/??",
    "14": "m/d/yy",
    "15": "d-mmm-yy",
    "16": "d-mmm",
    "17": "mmm-yy",
    "18": "h:mm AM/PM",
    "19": "h:mm:ss AM/PM",
    "20": "h:mm",
    "21": "h:mm:ss",
    "22": "m/d/yy h:mm",
    "30": "m/d/yy", // added fmtid 30 in _table
    "37": "#,##0 ;(#,##0)",
    "38": "#,##0 ;[Red](#,##0)",
    "39": "#,##0.00;(#,##0.00)",
    "40": "#,##0.00;[Red](#,##0.00)",
    "45": "mm:ss",
    "46": "[h]:mm:ss",
    "47": "mmss.0",
    "48": "##0.0E 0",
    "49": "@",
    "56": "\"上午/下午 \"hh\"時\"mm\"分\"ss\"秒 \"",
    "176": "[$-F800]dddd\\,\\ mmmm\\ dd\\,\\ yyyy"
}

cellText: false

// SSF._table
{
    "0": "General",
    "1": "0",
    "2": "0.00",
    "3": "#,##0",
    "4": "#,##0.00",
    "9": "0%",
    "10": "0.00%",
    "11": "0.00E 00",
    "12": "# ?/?",
    "13": "# ??/??",
    "14": "m/d/yy",
    "15": "d-mmm-yy",
    "16": "d-mmm",
    "17": "mmm-yy",
    "18": "h:mm AM/PM",
    "19": "h:mm:ss AM/PM",
    "20": "h:mm",
    "21": "h:mm:ss",
    "22": "m/d/yy h:mm",
    "37": "#,##0 ;(#,##0)",
    "38": "#,##0 ;[Red](#,##0)",
    "39": "#,##0.00;(#,##0.00)",
    "40": "#,##0.00;[Red](#,##0.00)",
    "45": "mm:ss",
    "46": "[h]:mm:ss",
    "47": "mmss.0",
    "48": "##0.0E 0",
    "49": "@",
    "56": "\"上午/下午 \"hh\"時\"mm\"分\"ss\"秒 \"",
    "176": "[$-F800]dddd\\,\\ mmmm\\ dd\\,\\ yyyy"
}

SSFImplicit already includes fmt30, but when cellText: false, it won't be used.