Hi,
Thanks for the file. But sadly I couldn't reproduce it here, it works fine here:

I wonder if this could be related with localization in some way. You are getting the unformatted date serial number (43496), which is what Excel show if you formatted the cell as "General"
But the cell is formatted as "dd", which should show the day. What locale is your machine in?
The weirdest part is that we didn't change that I remember anything that could be remotely changing this. If it doesn't work in 7.18 it shouldn't work in 7.17.
Can you run the following program and let me know the results in your machine?
program Project1;
{$APPTYPE CONSOLE}
{$R *.res}
uses
System.SysUtils,
FlexCel.VCLSupport,
FlexCel.Core,
FlexCel.XlsAdapter,
Windows;
begin
var Xls := TXlsFile.Create('.\2023.xlsx');
Xls.ActiveSheetByName := 'Februar';
WriteLn('GetUserDefaultLCID: ', GetUserDefaultLCID);
WriteLn('GetThreadLocale: ', GetThreadLocale);
WriteLn('XF: ', Xls.GetCellFormat(5, 2));
WriteLn('Format: ', Xls.GetFormatString(Xls.GetCellFormat(5, 2)));
WriteLn(Xls.GetStringFromCell(5, 1).ToString);
WriteLn(Xls.GetStringFromCell(5, 2).ToString);
ReadLn;
end.
For what it is worth, in my machine I get:
