For many years I have been looking at LibreOffice as the alternative to MS Office and I cannot understand why you never correct the date format problem! LibreOffice must accept by default the system date format and not something completely different based on local language and even worse you are using a wrong format for Portugal, as what we use is DD/MM/YY, or DD/MM/YYYY (some times DD-MM-YY can be used but it’s not official format).
Also some basic things on Calc are not in an excel when we open in Calc, and dates are always wrong.
Please make LO to accept the language formats of the system, like MS Office.
Are you sure you configured your computer correctly?
You didn’t mention your OS nor your LO version.
LO does retrieve the default system language formats unless you changed something in Tools>Options, Language Settings>Languages. Check there first.
Please do not use Add Answer but edit your original question to enhance the details of your question (answers are reserved for solutions to a problem on this Q&A site).
Please make LO to accept the language formats of the system, like MS Office.
You are addressing wrong audience with this request. We are users like you and instead of complaining about issues, which nobody can reproduce, since they are not underpinned by any facts, tell us the problem. And if you want a software equivalently working like Excel … use Excel.
Just set my language to `Portuguese (Portugal)` and see that date acceptance pattern is indeed set to `D-M-Y;D-M`. So you may want to add `D/M/Y` here (i.e. `Tools -> Options -> Language Settings -> Languages -> Input field: Date acceptance patterns:`), to correctly recognize your preferences.
Please keep in mind: Sometimes official rules released by national standard bureaus differs from what is common practice for traditional and historic reasons.
LibreOffice must accept by default the system date format and not something completely different based on local language
I assume you ask why users here haven’t yet implemented tdf#73242. I have good news for you: since you are now also a user of this site, you have the same opportunity to fix this as anyone who you addressed in your question 
and even worse you are using a wrong format for Portugal, as what we use is DD/MM/YY, or DD/MM/YYYY (some times DD-MM-YY can be used but it’s not official format)
If you know a normative source for your claim, please file a bug report, like tdf#119613.