I’m doing a list of recurring (yearly) events from 1/jan to 31/dic, without the year: insurance expirations.
If I insert 1/1 and the format cell adjusts to 1/jan but the cell value is also adjusted to 01/01/2018.
I need the cell values to be 01/01 to prevent sorting problem when I put new line the next years.
A date is a single point in time. So, simply recurring “Jan 1st” is not a date, but some kind of abstraction. LibreOffice doesn’t deal with abstractions; its dates always include years (and also times). You may decide to hide years (using relevant format string), but the years will still be there, hidden, and waiting to take part in sorting process.
So, if you really need that “incomplete” dates, then use text, not date.
I figured it after all my researches in internet.
Thanks by the way. 
Then how I can set a default year for every date I put, for example: if I write 1/1 then the calc put 01/01/2000, if I write 3/2 then the calc put 3/2/2000.
LibreOffice chooses current year IIRC. I don’t think there’s a configuration for that.
I solved the problem making a column with =TEXT(G3;“DD/MMM”) where G3 is the cell with the date (f. e. 5/feb/2018) and the result is a value 05/feb without year.