UPDATE[docs]: Compatibility and other engines

docs/src/.vitepress/config.mts

@@ -75,9 +75,13 @@ export default defineConfig({
             link: "/features/units",
           },		  
           {
-            text: "Serial Numbers",
+            text: "Dates and serial numbers",
             link: "/features/serial-numbers",
           },
+          {
+            text: "Numbers in IronCalc",
+            link: "/features/numbers-in-ironcalc",
+          },		  
           {
             text: "Unsupported Features",
             link: "/features/unsupported-features",
@@ -2028,6 +2032,16 @@ export default defineConfig({
           },
         ],
       },
+      {
+        text: "Spreadhsheet Engines",
+        collapsed: true,
+        items: [
+          {
+            text: "Spreadsheet Engines",
+            link: "/other-spreadsheets/index",
+          },
+        ],
+      },
       {
         text: "Contributing",
         collapsed: true,

docs/src/features/numbers-in-ironcalc.md

@@ -0,0 +1,38 @@
+---
+layout: doc
+outline: deep
+lang: en-US
+---
+
+# Numbers in IronCalc
+::: warning
+**Note:** This draft page is under construction 🚧
+
+::: warning
+**Note:** This page contains technical documentation
+
+Numbers in IronCalc are [IEE 754](https://en.wikipedia.org/wiki/IEEE_754) doubles (64 bit) and are displayed uo to 15 decimal digits.
+
+## Integers
+
+Some Integers are well represented by IEEE 754 doubles. The largest integer that can be stored perfectly as a double is:
+
+$$
+2^53 = 9,007,199,254,740,992
+$$
+
+## Floating points
+
+The reader should be aware that numbers like 0.1 or 0.3 are not stored perfectly by computers, _only an approximation to them_ is stored.
+This results in imperfect operations like the famous `0.1 + 0.2 != 0.3`.
+
+When comparing numbers we also compare up to 15 significant figures. With this 'trick' `=IF(0.2+0.1=0.3,TRUE,FALSE)` is actually `TRUE`.
+
+
+
+## Compatibility issues
+
+Excel [mostly follows IEEE 754](https://learn.microsoft.com/en-us/office/troubleshoot/excel/floating-point-arithmetic-inaccurate-result). Like IronCalc displays numbers with 15 significant digits. Excel does a few other undisclosed 'hacks'.
+If the result of an addition (or subtraction) of two non very small numbers is a number close to EPS and it is the end of the calculation then it is zero.
+
+That's is how it gets `=0.3-0.2-0.1` as `0`. However `=1*(0.3-0.2-0.1)` in Excel is `-2.77556E-17`

docs/src/features/serial-numbers.md

@@ -31,3 +31,20 @@ The fractional part of a serial number represents time, as a fraction of the day
 Since date-times are stored as numbers, they can be used for arithmetic operations in formulas. For example, it is possible to determine the difference between two dates by subtracting one serial number from the other.
 
 **Note**: A #VALUE! error is reported if a date-formatted cell contains a number less than 1 or greater than 2,958,465.
+
+## Compatibility Notes
+
+Excel has an infamous [feature](https://learn.microsoft.com/en-us/office/troubleshoot/excel/wrongly-assumes-1900-is-leap-year) that was ported from a bug in Lotus 1-2-3 that assumes that the year 1900 is a leap year.
+
+That means that serial numbers 1 to 60 in IronCalc are different than Excel.
+
+In IronCalc, Google Sheets, Libre Office and Zoho Date(1900,1,1) returns 2
+
+In Excel Date(1900,1,1) returns 1.
+
+Gnumeric solves the problem in yet another way. It follows Excel from 1 to 59, skips 60, and it follows Excel (and all other engines from there on).
+A formula like `=DAY(60)` produces `#NUM!` in Gnumeric.
+
+Serial number 61 corresponds to 1 March 1900, and from there on most spreadsheet engines agree.
+
+IronCalc, like Excel, doesn't deal with serial numbers outside of the range [1, 2,958,465]. Other engines like Google sheets, do not have an upper limit.

docs/src/functions/date_and_time/day.md

@@ -7,28 +7,42 @@ lang: en-US
 ::: warning
 **Note:** This draft page is under construction 🚧
 :::
+
 ## Overview
 DAY is a function of the Date and Time category that extracts the day of the month from a valid date [serial number](/features/serial-numbers.md), returning a number in the range [1, 31].
+
 ## Usage
 ### Syntax
 **DAY(<span title="Number" style="color:#1E88E5">date</span>) => <span title="Number" style="color:#1E88E5">day</span>**
+
 ### Argument descriptions
-* *date* ([number](/features/value-types#numbers), required). The date for which the day of the month is to be calculated, expressed as a [serial number](/features/serial-numbers.md) in the range [0, 2958465]. The value 0 corresponds to the date 1899-12-30, while 2958465 corresponds to 9999-12-31.
+* *date* ([number](/features/value-types#numbers), required). The date for which the day of the month is to be calculated, expressed as a [serial number](/features/serial-numbers.md) in the range [1, 2958465]. The value  corresponds to the date 1899-12-31, while 2958465 corresponds to 9999-12-31.
+
 ### Additional guidance
 If the supplied _date_ argument has a fractional part, DAY uses its [floor value](https://en.wikipedia.org/wiki/Floor_and_ceiling_functions).
+
 ### Returned value
 DAY returns an integer [number](/features/value-types#numbers) in the range [1, 31], that is the day of the month according to the [Gregorian calendar](https://en.wikipedia.org/wiki/Gregorian_calendar).
+
 ### Error conditions
 * In common with many other IronCalc functions, DAY propagates errors that are found in its argument.
 * If no argument, or more than one argument, is supplied, then DAY returns the [`#ERROR!`](/features/error-types.md#error) error.
 * If the value of the *date* argument is not (or cannot be converted to) a [number](/features/value-types#numbers), then DAY returns the [`#VALUE!`](/features/error-types.md#value) error.
 * For some argument values, DAY may return the [`#DIV/0!`](/features/error-types.md#div-0) error.
-* If date is less than 0, or greater than 2,958,465, then DAY returns the [`#NUM!`](/features/error-types.md#num) error.
+* If date is less than 1, or greater than 2,958,465, then DAY returns the [`#NUM!`](/features/error-types.md#num) error.
 * At present, DAY does not accept a string representation of a date literal as an argument. For example, the formula `=DAY("2024-12-31")` returns the [`#VALUE!`](/features/error-types.md#value) error.
 <!--@include: ../markdown-snippets/error-type-details.txt-->
+
 ## Details
 IronCalc utilizes Rust's [chrono](https://docs.rs/chrono/latest/chrono/) crate to implement the DAY function.
+
+## Compatibility Notes
+
+In IronCalc the argument of DAY cannot be text. This is confusing, and error prone, as in "10/12/2026" it is not clear if the day is 10 or 12.
+Most of all other spreadsheet engines like Excel, Google Sheets, Libre Office and Gnumeric accept text as input.
+
 ## Examples
+
 [See some examples in IronCalc](https://app.ironcalc.com/?example=day).
 
 ## Links

docs/src/other-spreadsheets/index.md

@@ -0,0 +1,30 @@
+---
+layout: doc
+outline: deep
+lang: en-US
+---
+
+::: warning
+**Note:** This draft page is under construction 🚧
+:::
+
+# Other spreadsheet engines
+
+There are numerous spreadsheet engines out there
+
+## Excel, the Friendly Giant
+
+## Google Sheets
+
+## LibreOffice
+
+## Gnumeric
+
+
+# Links
+
+For a list of spreadsheet software you can consult [wikipedia](https://en.wikipedia.org/wiki/List_of_spreadsheet_software).
+
+
+
+