Calc does *not* filter the entry based on cell format string. When your input data does not match any numeric format, it is simply considered a text. This is the correct behavior, and should not change.

To filter input, there is a dedicated "validity" function [1], and you may set your cells values to match some criteria, like being between some date range (which would make sure that the input was numeric).

Closing NOTABUG.

[1] https://help.libreoffice.org/latest/en-US/text/scalc/guide/validity.html?DbPAR=CALC

An alternative option for someone who want to check if their entry is correct may be using conditional formatting [1] (e.g., marking cells not matching the condition red), or using value highlighting [2].

[1] https://help.libreoffice.org/latest/en-US/text/scalc/guide/cellstyle_conditional.html?DbPAR=CALC
[2] https://help.libreoffice.org/latest/en-US/text/scalc/01/03080000.html?DbPAR=CALC

(In reply to Mike Kaganski from comment #1)
> Calc does *not* filter the entry based on cell format string. When your
> input data does not match any numeric format, it is simply considered a
> text. This is the correct behavior, and should not change.
> 
> To filter input, there is a dedicated "validity" function [1], and you may
> set your cells values to match some criteria, like being between some date
> range (which would make sure that the input was numeric).
> 
> Closing NOTABUG.
> 
> [1]
> https://help.libreoffice.org/latest/en-US/text/scalc/guide/validity.
> html?DbPAR=CALC

"This is the correct behavior, and should not change." You HAVE to be kidding?!
Incorrect data being allowed IS a problem, and at the very lest should prompt a warning.

(In reply to Edward Redondo from comment #3)

> 
> "This is the correct behavior, and should not change." You HAVE to be
> kidding?!
> Incorrect data being allowed IS a problem, and at the very lest should
> prompt a warning.

"09/081/2021" is a valid text string an not incorrect data. Imagine you are a manufacturer and you spare parts are numbered this way. What bug report will you write if you
- every time you enter such value getting a warning that your part number (text) is an invalid date  --or--
- you get it turned into a date without notice

You have been explained that formatting a cell is not an *input* filter and that there is a functionality called "data validity" (Data -> Validity), which allows to create a customized validity check of data entered into cells.

In other words:

You need to understand, that cell format is *not* intended to define "what values a cell may contain"; it is *only* to define "what procedure to use when show the *existing* content of the cell on screen/on paper".

I.e., when you type characters in a cell, the format string does *NOT* (and is not designed to!) affect what you *may* enter into the cell (i.e., it is not a filter). *Any* cell in a spreadsheet, however formatted, may contain any data - it may be numeric, or textual; it may be a calculated formula returning text, or number, or error.

The formatting of a cell does participate in conversion of your typed text to the cell content. But its part is just to be a *hint* to the program what to attempt, not a filter: i.e., a date format of the active cell might give a hint in which order the date parts (year/month/day) might go in the input string. But if the hint derived from the format does not match the input, the *correct* behavior of the program is to fallback to the most reliable result: accept the typed characters not as some converted number (which dates are), but as plain text. There is *NO* "invalid" text: just anything that you may type is a valid text string, and must be accepted as a valid cell content.

What you are talking about is *validation* of input; and as Uwe and I informed you, there is a dedicated tool for *validation*. What you suggest is using a wrong tool for the task: you try to use cell format for the validation task, for which there is Validity.

Then another suggestion: Have formatting work like Spell-Checking, mark or highlight entries that do not match the cell (Calc) or text (Word) format?

My typo "9/081/2021" typo in a date-formatted cell would have the "081" in red telling me that is a wrong entry for the date format. Is that doable?

(In reply to Edward Redondo from comment #6)
> My typo "9/081/2021" typo in a date-formatted cell would have the "081" in
> red telling me that is a wrong entry for the date format. Is that doable?

No.

1. Format string does not limit the ways that you may *enter* the valid date. E.g., defining a date format like MM/DD/YY, you may enter "aug 1" (in a cell with en-US number format), and get it converted to a date; or you may enter "2021-09-09", and also get it converted. Or you may enter 10%, and the entry will get recognized. Or 1 2/3. And so on. All these will get recognized as a valid *number* (using different rules built into the program string parser), and will end up as valid numbers in the cell.

2. More importantly: each number format string may consist of *four* parts: for positive numbers; negative numbers; zero; and text. If you omit any part, it is *not* considered "absent", but a default value is implicitly used for the respective part. So your partial format string MM/DD/YYYY in reality is equal to something like "MM/DD/YYYY;MM/DD/YYYY;MM/DD/YYYY;@", where the last fourth part tells "show text as it was entered". So any text is *valid* from the format point of view. And there is no way to make it invalid. Any data *matches* any format string. And format string *has no idea* why the program decided that this cell is a text.

But you may use custom format string like e.g. "MM/DD/YYYY;MM/DD/YYYY;MM/DD/YYYY;[red]@", which would mark red all texts, regardless of the reason why it is text. You may learn more about format strings in help: https://help.libreoffice.org/latest/en-US/text/shared/01/05020301.html?&DbPAR=CALC

Thanks to all who replied.