In Qt5, the MMM format, which displays the month name in a short format (e.g., "Dec" instead of "12" or "December"), when using it for the Spanish locale, shows the abbreviated month name with a dot at the end, for example, for, "25th December 2016", in MMM/dd/yyyy format, shows "Dic./25/2016".
The problem is when editing. For displaying a date, it's ok, since it's what the Spanish rules say, but it isn't suitable for editing. I'm forced to position the cursor just after the point, at the end of the month part, to delete the point, and then, the rest of the month name. If I try to put the cursor just before the point, to edit just the month name part, the field is in "read-only" mode. I can't not remove anything, unless I remove everything from the very right of the line and back, character to character (or just selecting the whole month part and remove everything).
Besides, the point is not autocompleted. So, if the user writes the new month forgetting the dot, the edition is rejected and fall backs to the original value.
Taking all of it into account, editing a QDateEdit is a bit cumbersome (in Spanish).
I don't know if it is the built-in QDateEdit behaviour, or internally it is using a QRegExpValidator, but in that case, I don't know what it's the regexp expression, to personalize it from it.
In short, how can I "solved" it? (the cumbersome edition; it's ok for me both, removing the dot, or changing the validation).
The Qt 5 behavior is correct and there's nothing Qt can do (or should do). The only correct and acceptable short name of December in a es_ES locale is "dic.", trailing dot included. That's what CLDR says: