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Dates and Times

File Format Support (click to show)

Dates are a core concept in nearly every spreadsheet application in existence. Some legacy spreadsheet apps only supported dates. Others supported times as a distinct concept from dates.

Some file formats store dates in a textual format, while others store dates with numbers representing a difference from an epoch.

Many spreadsheet apps use special number formats to signal that values are dates or times. Quattro Pro for DOS had a distinct set of Date number formats and Time number formats, but did not have a mixed Date + Time format. OpenOffice uses ISO 8601 duration strings for pure time data.

Lotus 1-2-3 used a "1900" date system, while Numbers exclusively supports 1904 under the hood. Excel file formats typically include options for specifying the date system. OpenOffice can support arbitrary starting dates.

FormatsDateTimeD+TDate StorageDate System
NUMBERSNumber1904 Only
XLSX / XLSMNumber1900 + 1904
XLSX (Strict ISO)Relative Date1900 + 1904
XLSBNumber1900 + 1904
XLMLRelative Date1900 + 1904
XLS (BIFF5/8)Number1900 + 1904
XLS (BIFF2/3/4)Number1900 + 1904
XLR (Works)Number1900 + 1904
ET (WPS 电子表格)Number1900 + 1904
ODS / FODS / UOSISO Duration or DateArbitrary
CSV / TSV / TextPlaintextCalendar
DBF**Number or PlaintextCalendar
WKS (Works)Number1900

X (✕) marks features that are not supported by the file formats. For example, the WK1 file format had date-only formats and time-only formats but no mixed date-time formats.

Newer DBF levels support a special T field type that represents date + time.

The QPW file format supports mixed date + time formats in custom number formats.

Lotus 1-2-3, Excel, and other spreadsheet software do not have a true concept of date or time. Instead, dates and times are stored as offsets from an epoch. The magic behind date interpretations is hidden in functions or number formats.

SheetJS attempts to create a friendly JS date experience while also exposing options to use the traditional date codes.

Date and time handling was overhauled in version 0.20.0. It is strongly recommended to upgrade.

The following example exports the current time to XLSX spreadsheet. The time shown on this page will be the time displayed in Excel.

Live Editor
function SheetJSNow() {
  const [date, setDate] = React.useState(new Date());
  const xport = React.useCallback(() => {
    /* generate array of arrays */
    const aoa = [[date]];
    /* to avoid confusion, set milliseconds to 0 */
    /* generate workbook */
    const ws = XLSX.utils.aoa_to_sheet(aoa, {dense: true});
    /* set cell A1 number format */
    ws["!data"][0][0].z = "yyyy-mm-dd hh:mm:ss"
    ws["!cols"] = [{wch: 20}];

    /* generate workbook and export */
    const wb = XLSX.utils.book_new(ws, "Sheet1");
    XLSX.writeFile(wb, "SheetJSNow.xlsx");
  }, []);
  return ( <>
      <b>Local Time:</b>{date.toString()}
      <button onClick={()=>setDate(new Date())}>Refresh</button>
    <button onClick={xport}>Export XLSX</button>
  </> );

How Spreadsheets Understand Time

Excel stores dates as numbers. When displaying dates, the format code should include special date and time tokens like yyyy for long year. EDATE and other date functions operate on and return date numbers.

For date formats like yyyy-mm-dd, the integer part represents the number of days from a starting epoch. For example, the date 19-Feb-17 is stored as the number 42785 with a number format of d-mmm-yy.

The fractional part of the date code serves as the time marker. Excel assumes each day has exactly 86400 seconds. For example, the date code 0.25 has a time component corresponding to 6:00 AM.

For absolute time formats like [hh]:mm, the integer part represents a whole number of 24-hour (or 1440 minute) intervals. The value 1.5 in the format [hh]:mm is interpreted as 36 hours 0 minutes.

Date and Time Number Formats

Assuming a cell has a formatted date, re-formatting as "General" will reveal the underlying value. Alternatively, the TEXT function can be used to return the date code.

The following table covers some common formats:

Common Date-Time Formats (click to show)
yyShort (2-digit) year
yyyyLong (4-digit) year
mShort (1-digit) month
mmLong (2-digit) month
mmmShort (3-letter) month name
mmmmFull month name
mmmmmFirst letter of month name
dShort (1-digit) day of month
ddLong (2-digit) day of month
dddShort (3-letter) day of week
ddddFull day of week
hShort (1-digit) hours
hhLong (2-digit) hours
mShort (1-digit) minutes
mmLong (2-digit) minutes
sShort (1-digit) seconds
ssLong (2-digit) seconds
A/PMeridiem ("A" or "P")
AM/PMMeridiem ("AM" or "PM")

m and mm are context-dependent. It is interpreted as "minutes" when the previous or next date token represents a time (hours or seconds):

yyyy-mm-dd hh:mm:ss
^^ ^^
month minutes

mmm, mmmm, and mmmmm always represent months.

1904 and 1900 Date Systems

The interpretation of date codes requires a shared understanding of date code 0, otherwise known as the "epoch". Excel supports two epochs:

  • The default epoch is "January 0 1900". The 0 value is 00:00 on December 31 of the year 1899, but it is formatted as January 0 1900.

  • Enabling "1904 Date System" sets the default epoch to "January 1 1904". The 0 value is 00:00 on January 1 of the year 1904.

The workbook's epoch can be determined by examining the workbook's wb.Workbook.WBProps.date1904 property:

if(!(wb?.Workbook?.WBProps?.date1904)) {
/* uses 1904 date system */
} else {
/* uses 1900 date system */
Why does the 1904 date system exist? (click to show)

1900 was not a leap year. For the Gregorian calendar, the general rules are:

  • every multiple of 400 is a leap year
  • every multiple of 100 that is not a multiple of 400 is not a leap year
  • every multiple of 4 that is not a multiple of 100 is a leap year
  • all other years are not leap years.

Lotus 1-2-3 erroneously treated 1900 as a leap year. This can be verified with the @date function:

@date(0,2,28) -> 59    // Lotus accepts 2/28/1900
@date(0,2,29) -> 60 // <--2/29/1900 was not a real date
@date(0,2,30) -> ERR // Lotus rejects 2/30/1900

Excel extends the tradition in the default date system. The 1904 date system starts the count in 1904, skipping the bad date.

Relative Epochs

The epoch is based on the system timezone. The epoch in New York is midnight in Eastern time, while the epoch in Seattle is midnight in Pacific time.

This design has the advantage of uniform time display: "12 PM" is 12 PM irrespective of the timezone of the viewer. However, this design precludes any international coordination (there is no way to create a value that represents an absolute time) and makes JavaScript processing somewhat ambiguous (since JavaScript Date objects are timezone-aware)

This is a deficiency of the spreadsheet software. Excel has no native concept of universal time.

How Files Store Dates and Times

Technical Details (click to show)

XLS, XLSB, and most binary formats store the raw date codes. Special number formats are used to indicate that the values are intended to be dates/times.

CSV and other text formats typically store actual formatted date values. They are interpreted as dates and times in the user timezone.

XLSX actually supports both! Typically dates are stored as n numeric cells, but the format supports a special type d where the data is an ISO 8601 date string. This is not used in the default Excel XLSX export and third-party support is poor.

ODS does support absolute time values but drops the actual timezone indicator when parsing. In that sense, LibreOffice follows the same behavior as Excel.

Numbers uses a calendar date system, but records pure time values as if they are absolute times in 1904 January 01. It is spiritually equivalent to the 1904 mode in Excel and other spreadsheet applications.

How JavaScript Engines Understand Time

JavaScript provides a Date object which represents an absolute time. Under the hood, Date uses the "UNIX" epoch of 1970 January 01 midnight in UTC. This means the actual zero date is different in different timezones!

LocationIANA Timezonenew Date(0) in local time
HonoluluPacific/Honolulu1969-12-31 02:00 PM
Los AngelesAmerica/Los_Angeles1969-12-31 04:00 PM
New YorkAmerica/New_York1969-12-31 07:00 PM
Sao PauloAmerica/Sao_Paulo1969-12-31 09:00 PM
LondonEurope/London1970-01-01 01:00 AM
CairoAfrica/Cairo1970-01-01 02:00 AM
DjiboutiAfrica/Djibouti1970-01-01 03:00 AM
ChennaiAsia/Kolkata1970-01-01 05:30 AM
ShanghaiAsia/Shanghai1970-01-01 08:00 AM
SeoulAsia/Seoul1970-01-01 09:00 AM
SydneyAustralia/Sydney1970-01-01 10:00 AM

In modern environments, the IANA Timezone and timezone offset can be discovered through the Intl and Date objects:

Live Editor
function LocalInfo() {
  const date = new Date();
  return ( <>
  <b>Local Time</b>: {date.toString()}<br/>
  <b>Time offset (relative to UTC)</b>: {-date.getTimezoneOffset()/60} hours <br/>
  <b>IANA Timezone</b>: {Intl.DateTimeFormat().resolvedOptions().timeZone}

The timezone information is provided by the JavaScript engine and local settings. There are outstanding Google Chrome and V8 bugs related to rounded offsets for timezones under a lunar calendar. The last timezone to switch to the Gregorian calendar was Africa/Monrovia (in 1972).

SheetJS utilities attempt to work around the browser bugs.

UTC and Local Time

The Date object has a number of prototype methods for inspecting the object. Some methods interact with the true value, while others convert to the local timezone. Some methods are listed in the table below:

FeatureLocal Time methodUTC method
Month (0-11)getMonthgetUTCMonth
Day of the monthgetDategetUTCDate
Entire datetoStringtoUTCString

It is typical for websites and other applications to present data in local time. To serve an international audience, backend servers typically use UTC time.

The following example shows the time when the page was loaded. The same absolute time will appear to be different under local and UTC interpretations:

Live Editor
function LocalUTC() {
  const d = new Date();
  /* display number with 2 digits, prepending `0` if necessary */
  const f = (n) => n.toString().padStart(2, "0");
  /* HH:MM:SS using local interpretation */
  const local = `${f(d.getHours())}:${f(d.getMinutes())}:${f(d.getSeconds())}`;
  /* HH:MM:SS using UTC interpretation */
  const utc = `${f(d.getUTCHours())}:${f(d.getUTCMinutes())}:${f(d.getUTCSeconds())}`;
  return ( <>
    <b>Local Interpretation</b><br/>
    <code>toString</code>: {d.toString()}<br/>
    24-hour time: {local}<br/>
    <b>UTC Interpretation</b><br/>
    <code>toUTCString</code>: {d.toUTCString()}<br/>
    24-hour time: {utc}<br/>

How SheetJS handles Dates and Times

SheetJS attempts to reconcile the spreadsheet and JavaScript date concepts.

The default behavior for all parsers is to generate number cells. Setting cellDates to true will force the parsers to store dates.

Live Editor
function SheetJSCellDates() {
  var csv = "Date,10/6/2048";

  // cell B1 will be { t: 'n', v: 54337 }
  var wb_sans_date =, {type:"binary"});
  var ws_sans_date = wb_sans_date.Sheets.Sheet1;

  // cell B1 will be { t: 'd', v: <Date: 2048-10-06 00:00:00 UTC> }
  var wb_with_date =, {type:"binary", cellDates: true});
  var ws_with_date = wb_with_date.Sheets.Sheet1;

  return (<>
  <b>Cell B1:</b><br/><br/>
      <td><code>{ws_with_date["B1"].v.toISOString()}</code> (Date object)</td>

When writing, date cells are automatically translated back to numeric cells with an appropriate number format.

The value formatting logic understands date formats and converts when relevant. It always uses the UTC interpretation of Date objects.

Date Objects

The actual values stored in cells are intended to be correct when interpreted using UTC date methods.

For example, DateTime.xlsx is a test file with the following data:

DateTime2048-10-06 15:00:00

The raw data values are shown in the live demo. The UTC date string will show the same value as Excel irrespective of the local timezone.

Live Editor
function SheetJSDateTimeXlsxValues() {
  const [data, setData] = React.useState([[]]);
  React.useEffect(() => { (async() => {
    const ab = await (await fetch("/DateTime.xlsx")).arrayBuffer();
    const wb =, {cellDates: true, dense: true});
  })(); });
  return ( <table><thead>
    <th>Excel Date</th><th>UTC Date</th><th>Local Date</th>
    {data.slice(1).map((row,R) => ( <tr key={R}>
    </tr> ))}
  </tbody></table> );

Utility Functions

Utility functions that deal with JS data accept a cellDates argument which dictates how dates should be handled.

Functions that create a worksheet will adjust date cells and use a number format like m/d/yy to mark dates:

// Cell A1 will be a numeric cell whose value is the date code
var ws = XLSX.utils.aoa_to_sheet([[new Date()]]);

// Cell A1 will be a date cell
var ws = XLSX.utils.aoa_to_sheet([[new Date()]], { cellDates: true });

Functions that create an array of JS objects with raw values will keep the native representation:

// Cell A1 is numeric -> output is a number
var ws = XLSX.utils.aoa_to_sheet([[new Date()]]);
var A1 = XLSX.utils.sheet_to_json(ws, { header: 1 })[0][0];

// Cell A1 is a date -> output is a date
var ws = XLSX.utils.aoa_to_sheet([[new Date()]], { cellDates: true });
var A1 = XLSX.utils.sheet_to_json(ws, { header: 1 })[0][0];

UTC Option

Some API functions support the UTC option to control how dates are handled.


If UTC is true, the dates will be correct when interpreted in UTC. By default, the dates will be correct when interpreted in local time.

Typically UTC is used for data from an API endpoint, as servers typically emit UTC dates and expect scripts to localize. The local interpretation is sensible when users submit data, as they will be providing times in their local timezone.

aoa_to_sheet / sheet_add_aoa / json_to_sheet / sheet_add_json

If UTC is true, the UTC interpretation of dates will be used.

Typically UTC is used for data from an API endpoint, as servers typically emit UTC dates and expect scripts to localize. The local interpretation is sensible when date objects are generated in the browser.

table_to_book / table_to_sheet / sheet_add_dom

If UTC is true, potential dates are interpreted as if they represent UTC times. By default, potential dates are interpreted in local time.

Typically UTC is used for data exported from Excel or other spreadsheet apps. If the table is programmatically generated in the frontend, the dates and times will be in the local timezone and the local interpretation is preferable.

Number Formats

By default, the number formats are not emitted. For Excel-based file formats, passing the option cellNF: true adds the z field.

The helper function XLSX.SSF.is_date parses formats and returns true if the format represents a date or time:

XLSX.SSF.is_date("yyyy-mm-dd"); // true

XLSX.SSF.is_date("0.00"); // false
Live Demo (click to show)
Live Editor
function SSFIsDate() {
  const [format, setFormat] = React.useState("yyyy-mm-dd");
  const cb = React.useCallback((evt) => {
  const is_date = XLSX.SSF.is_date(format);
  return ( <>
    <div>Format <b>|{format}|</b> is {is_date ? "" : "not"} a date/time</div>
    <input type="text" onChange={cb}/>
  </> );

How JSON and APIs Understand Time

JSON does not have a native representation for JavaScript Date objects.

Starting from a Date object, the JSON.stringify method will encode the object as a ISO 8601 date string. Applying JSON.parse to the result will return the string rather than a proper Date object.

SheetJS utility functions will not try to interpret those strings as dates. Instead, the strings will be translated to text.

In the following example, the "ISO Text" data will be converted to a string cell while the "Date Obj" data will be converted to a spreadsheet date.

Live Editor
function SheetJSONDates() { return ( <button onClick={() => {
  const aoa = [
    ["ISO Text", "2001-01-01T00:00:00.000Z"],          // B1 will be text
    ["Date Obj", new Date("2001-01-01T00:00:00.000Z")] // B2 will be a date
  const ws = XLSX.utils.aoa_to_sheet(aoa);
  const wb = XLSX.utils.book_new(ws, "Data");
  XLSX.writeFile(wb, "SheetJSONDates.xlsx");
}}>Click to Export Sample Data</button> ); }

Many API wrapper libraries return dates as strings instead of Date objects.

If the SheetJS operations generate string cells, review the documentation for the wrapper library to ensure the other library is properly handling dates.

Fixing Arrays of Objects

Many APIs will return data as JSON objects. When particular fields are known to contain date strings, they can be manually fixed.

For example, the Export Tutorial makes a final array of objects with birthdays stored as strings:

const rows = [
{ name: "George Washington", birthday: "1732-02-22" },
{ name: "John Adams", birthday: "1735-10-19" },
// ... one row per President

If this dataset is exported, the birthday column will contain raw text values. A single Array#map operation can create a fixed dataset:

const new_rows ={birthday,}) => ({birthday: new Date(birthday),}))

The Date constructor interprets the dates in local time.

Excel and other spreadsheet software do not typically support dates before 1900. If there are dates before the threshold, it is strongly recommended to pass strings instead of Date objects.

JavaScript string to Date conversion is "implementation-dependent" and may misinterpret some date formats. When designing APIs, it is strongly recommended to pass ISO 8601 strings when possible.