Open the Calc and type a number ending with comma. For example 13. If You press
Enter, the number is automatically changed to date.

Before typing the number, the cell is of type "number". After pressing enter,
the type changes to "date".

If the number is greater than 31, it stays there as entered and type of cell
stays "number".

The behavior is not OK. OOo should act in consistent way.

Number ended by comma is often used to represent some numbering etc. If user
enters number to cell of type "number", I think it's likely that he means "number".
It's less probable that he enters number to "number" typed cell and he means a
date from current month and year.

Autocomplete shouldn't by default create date from number itself in "number" cell.

If user needed to enter a date into cell, he should change type of cell and then
enter the date into. In such case, autocomplete becomes very handy indeed.
Or if he enters for example "13.5", it's OK that autocomplete generates a date
from that. But not from number itself, and even split behavior on number 31.