Created attachment 190445 [details]
Filled in on transfer through MariaDB connector C to Base.

Filled in on transfer through MariaDB connector C to Base. 1.1.1800 apeared only, when transfering with empty date fields. When you consequently create new rew in Base, date fields stay empty.

@Petr: are these date fields defined as "DEFAULT NULL" in the database, or otherwise defined as NULL being allowable ?

Is the default LO configuration base date stored in the database, or does it just appear in the form display ?

For example, in the table containing the dates, does the data entry grid view also show the default LO date for NULL date fields ? When you navigate from one data entry to the next in any given tuple, does the default LO date get written to the database server ?

For example, I don't reproduce the described behaviour with a form I created many years ago, containing several date fields which can be NULL, and have no default value set, in 

Version: 7.6.2.1 (AARCH64) / LibreOffice Community
Build ID: 56f7684011345957bbf33a7ee678afaf4d2ba333
CPU threads: 8; OS: Mac OS X 14.0; UI render: Skia/Raster; VCL: osx
Locale: fr-FR (fr_FR.UTF-8); UI: fr-FR
Calc: threaded

against a MySQL Server 
8.0.27 (MySQL Community Server - GPL)

using the MariaDB C native connector.

(In reply to Alex Thurgood from comment #3)
> For example, I don't reproduce the described behaviour with a form I created
> many years ago, containing several date fields which can be NULL, and have
> no default value set, in 
> 
> Version: 7.6.2.1 (AARCH64) / LibreOffice Community
> Build ID: 56f7684011345957bbf33a7ee678afaf4d2ba333
> CPU threads: 8; OS: Mac OS X 14.0; UI render: Skia/Raster; VCL: osx
> Locale: fr-FR (fr_FR.UTF-8); UI: fr-FR
> Calc: threaded
> 
> against a MySQL Server 
> 8.0.27 (MySQL Community Server - GPL)
> 
> using the MariaDB C native connector.

I also created a new form against the same database, to see whether your problem might be due to a change in the way forms are created, but there was still no difference, my NULL date fields remain empty in the form until I add a date.

Created attachment 190451 [details]
Images from MariaDB and Base

0000-00-00 filed probably during transfer from Libre Office Calc...

Thank you for your response. 
Perhaps it's not a bug. I don't know. I looked into the MariaDB database and Base again and noticed that null fields really display nothing. When I had created a new row in the table, as seen in photo, 1/24 and 2/24 rows, the date field remained empty, and in MariaDB, the field is marked as NULL. I converted tables from the original Base internal database to MariaDB using Calc, and it seems that empty fields were replaced with the form "0000-00-00," which generates a minimum date of January 1, 1800 in the cells. Whether this is a bug, you will have to assess. When I converted data in the same way and used the Base ODBC connector to access MariaDB, the fields remained empty. I dont chect at that time if the fields were 0000-00-00 becouse there was no probem...

(In reply to Petr Poruban from comment #6)


> converted tables from the original Base internal database to MariaDB using
> Calc, and it seems that empty fields were replaced with the form
> "0000-00-00," which generates a minimum date of January 1, 1800 in the
> cells. Whether this is a bug, you will have to assess. When I converted data
> in the same way and used the Base ODBC connector to access MariaDB, the
> fields remained empty. I dont chect at that time if the fields were
> 0000-00-00 becouse there was no probem...

Data Migration
The now infamous Calc drag'n'drop / copy-paste to Base wizard is a well known culprit for screwing things up, unfortunately.

The situation appears to have been compounded here because your NULL DATE fields seem to have been represented initially as "0000-00-00" when you copied the data into Calc. Calc's date converter unit converts "0000-00-00" to the default date from which all dates are counted, as defined under LibreOffice > Preferences > Calc > Calculations.

So, when you copy data via Calc into a database table, the wizard takes the default zero date as the date to be inserted in the mariadb table. Not pretty, and probably should be considered a bug, IMHO.

I have a vague recollection that this might already have been flagged up in another bug report, but would have to go looking for it.

DATE DISPLAY IN A CONTROL (grid view, table data entry mode, etc)
There could also be a separate bug in the display of the date in the grid control with regard to unfilled date values, leading to a default proposed display of 01/01/1900. Currently, there is already a bug report about Mariadb connections showing data from the previous tuple in a table grid view when entering a new tuple / resultset, but that displayed data isn't saved to the table.

See, for example:

https://bugs.documentfoundation.org/show_bug.cgi?id=155529#c4

Wondering if the culprit is in:

formattedcolumnvalue.cxx

lines 78-79:

FormattedColumnValue_Data()
            :m_aNullDate( DBTypeConversion::getStandardDate() )

@Pter: This is probably actually a duplicate of the underlying behavior exposed in bug 155529.

Unfortunately, without a detailed step-by-step description of how to reproduce the reported problem here, you bug report has been closed.

If you could provide such a detailed description, the bug report could be reopened as Unconfirmed, for further testing.