That is because it is disabled in the message file with $datePreferences = false;

So do we want to change anything here? This setting has been in the Korean messages file for ages (see r15823 - 2006-07-26).

AheunHana wrote:

I have recently edited a help page of preferences on ko Wiktionary. In the process, I had found that the "Datetime" message on ko is divergent from other wikis. I think there is not an obstacle by changing it.

$datePreferences = false; removed in r41135.