forked from sheetjs/docs.sheetjs.com
dates
This commit is contained in:
parent
785679a682
commit
8907edf5d7
@ -76,6 +76,7 @@ store ISO 8601 Date strings like you would get from `date.toISOString()`. On
|
||||
the other hand, writers and exporters should be able to handle date strings and
|
||||
JS Date objects. Note that Excel disregards timezone modifiers and treats all
|
||||
dates in the local timezone. The library does not correct for this error.
|
||||
Dates are covered in more detail [in the Dates section](./features/dates)
|
||||
|
||||
Type `s` is the String type. Values are explicitly stored as text. Excel will
|
||||
interpret these cells as "number stored as text". Generated Excel files
|
||||
@ -86,44 +87,3 @@ have no assigned value but hold comments or other metadata. They are ignored by
|
||||
the core library data processing utility functions. By default these cells are
|
||||
not generated; the parser `sheetStubs` option must be set to `true`.
|
||||
|
||||
|
||||
#### Dates
|
||||
|
||||
<details>
|
||||
<summary><b>Excel Date Code details</b> (click to show)</summary>
|
||||
|
||||
By default, Excel stores dates as numbers with a format code that specifies date
|
||||
processing. For example, the date `19-Feb-17` is stored as the number `42785`
|
||||
with a number format of `d-mmm-yy`. The `SSF` module understands number formats
|
||||
and performs the appropriate conversion.
|
||||
|
||||
XLSX also supports a special date type `d` where the data is an ISO 8601 date
|
||||
string. The formatter converts the date back to a number.
|
||||
|
||||
The default behavior for all parsers is to generate number cells. Setting
|
||||
`cellDates` to true will force the generators to store dates.
|
||||
|
||||
</details>
|
||||
|
||||
<details>
|
||||
<summary><b>Time Zones and Dates</b> (click to show)</summary>
|
||||
|
||||
Excel has no native concept of universal time. All times are specified in the
|
||||
local time zone. Excel limitations prevent specifying true absolute dates.
|
||||
|
||||
Following Excel, this library treats all dates as relative to local time zone.
|
||||
|
||||
</details>
|
||||
|
||||
<details>
|
||||
<summary><b>Epochs: 1900 and 1904</b> (click to show)</summary>
|
||||
|
||||
Excel supports two epochs (January 1 1900 and January 1 1904).
|
||||
The workbook's epoch can be determined by examining the workbook's
|
||||
`wb.Workbook.WBProps.date1904` property:
|
||||
|
||||
```js
|
||||
!!(((wb.Workbook||{}).WBProps||{}).date1904)
|
||||
```
|
||||
|
||||
</details>
|
||||
|
@ -1,3 +1,7 @@
|
||||
---
|
||||
sidebar_position: 2
|
||||
---
|
||||
|
||||
# Hyperlinks
|
||||
|
||||
<details>
|
||||
|
235
docz/docs/07-csf/07-features/03-dates.md
Normal file
235
docz/docs/07-csf/07-features/03-dates.md
Normal file
@ -0,0 +1,235 @@
|
||||
---
|
||||
sidebar_position: 3
|
||||
---
|
||||
|
||||
# Dates and Times
|
||||
|
||||
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
|
||||
|
||||
|
||||
## How Spreadsheets Understand Time
|
||||
|
||||
Excel stores dates as numbers. When displaying dates, the format code should
|
||||
include special date and time tokens like `yyyyy` 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:
|
||||
|
||||
<details open><summary><b>Common Date-Time Formats</b> (click to hide)</summary>
|
||||
|
||||
| Fragment | Interpretation |
|
||||
|:---------|:-----------------------------|
|
||||
| `yy` | Short (2-digit) year |
|
||||
| `yyyy` | Long (4-digit) year |
|
||||
| `m` | Short (1-digit) month |
|
||||
| `mm` | Long (2-digit) month |
|
||||
| `mmm` | Short (3-letter) month name |
|
||||
| `mmmm` | Full month name |
|
||||
| `mmmmm` | First letter of month name |
|
||||
| `d` | Short (1-digit) day of month |
|
||||
| `dd` | Long (2-digit) day of month |
|
||||
| `ddd` | Short (3-letter) day of week |
|
||||
| `dddd` | Full day of week |
|
||||
| `h` | Short (1-digit) hours |
|
||||
| `hh` | Long (2-digit) hours |
|
||||
| `m` | Short (1-digit) minutes |
|
||||
| `mm` | Long (2-digit) minutes |
|
||||
| `s` | Short (1-digit) seconds |
|
||||
| `ss` | Long (2-digit) seconds |
|
||||
| `A/P` | Meridien ("A" or "P") |
|
||||
| `AM/PM` | Meridien ("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
|
||||
```
|
||||
|
||||
</details>
|
||||
|
||||
### 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:
|
||||
|
||||
```js
|
||||
if(!!(((wb.Workbook||{}).WBProps||{}).date1904)) {
|
||||
/* uses 1904 date system */
|
||||
} else {
|
||||
/* uses 1900 date system */
|
||||
}
|
||||
```
|
||||
|
||||
:::note Why does the 1904 date system exist?
|
||||
|
||||
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.
|
||||
|
||||
The library attempts to normalize the dates. All times are specified in the
|
||||
local time zone. SheetJS cannot magically fix the technical problems with
|
||||
Excel and other spreadsheet software, but this represents .
|
||||
|
||||
|
||||
## How Files Store Dates and Times
|
||||
|
||||
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 plaintext 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.
|
||||
|
||||
|
||||
## How SheetJS handles Dates and Times
|
||||
|
||||
The default behavior for all parsers is to generate number cells. Passing the
|
||||
`cellDates` to true will force the parsers to store dates:
|
||||
|
||||
```js
|
||||
// cell A1 will be { t: 'n', v: 44721 }
|
||||
var wb_sans_date = XLSX.read("6/9/2022", {type:"binary"});
|
||||
|
||||
// cell A1 will be { t: 'd', v: <Date object representing June 9 2022> }
|
||||
var wb_with_date = XLSX.read("6/9/2022", {type:"binary", cellDates: true});
|
||||
```
|
||||
|
||||
When writing, date cells are automatically translated back to numeric cells
|
||||
with an appropriate number format.
|
||||
|
||||
The actual values stored in cells are intended to be correct from the
|
||||
perspective of an Excel user in the current timezone.
|
||||
|
||||
The value formatter understands date formats and converts when relevant.
|
||||
|
||||
### 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:
|
||||
|
||||
```js
|
||||
// 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:
|
||||
|
||||
```js
|
||||
// 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];
|
||||
```
|
||||
|
||||
### 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:
|
||||
|
||||
```js
|
||||
XLSX.SSF.is_date("yyyy-mm-dd"); // true
|
||||
|
||||
XLSX.SSF.is_date("0.00"); // false
|
||||
```
|
||||
|
||||
<details><summary><b>Live Demo</b> (click to show)</summary>
|
||||
|
||||
```jsx live
|
||||
function SSFIsDate() {
|
||||
const [format, setFormat] = React.useState("yyyy-mm-dd");
|
||||
const cb = React.useCallback((evt) => {
|
||||
setFormat(evt.target.value);
|
||||
});
|
||||
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}/>
|
||||
</>)
|
||||
}
|
||||
```
|
||||
|
||||
</details>
|
Loading…
Reference in New Issue
Block a user