Description:
I write on one cell the value '13:30:00' and on the cell below the value '13:50:00'. I select both cells and drag down the fill handle. I would expect the time "14:10:00" to appear on the third cell.
Instead, I find the second cell modified with the value "13:49:59" and on the third cell the value "14:09:59", and so on all the others with one second less.

Steps to Reproduce:
1. Write on cell 'A1' the value '13:30:00'.
2. Write on cell 'A2' the value '13:50:00'.
3. Select cells 'A1' and 'A2' and drag the fill handle down over several cells.


Actual Results:
Cell 'A2' changes its value to '13:49:59'.
The other cells below all have one second less: "14:09:59", "14:29:59", "14:49:59" and so on.

Expected Results:
Cell 'A2' was to remain with the value '13:50:00'.
The other cells below were to have the values '14:10:00', '14:30:00', '14:50:00' and so on.


Reproducible: Always


User Profile Reset: No

Additional Info:
[Information automatically included from LibreOffice]
Locale: it
Module: SpreadsheetDocument
[Information guessed from browser]
OS: Windows (All)
OS is 64bit: no
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I assume 'Firefox Browser 64 bit' incorrectly detects that my operating system is 32-bit. Instead, it is 64-bit (as 'Help - About LibreOffice' correctly detects).

Version: 7.5.3.2 (X86_64) / LibreOffice Community
Build ID: 9f56dff12ba03b9acd7730a5a481eea045e468f3
CPU threads: 8; OS: Windows 10.0 Build 19045; UI render: Skia/Raster; VCL: win
Locale: it-IT (it_IT); UI: it-IT
Calc: threaded