This is the 10772nd most frequent Polish word.
dwunaste
"Twelfth."
Here, 'dwunaste' is used as an ordinal number describing the specific day of December.
Dziś jest dwunaste grudnia.
Today is the twelfth of December.
In this case, 'dwunastej' is the locative case of 'dwunasta', specifying the hour of the day.
Mam wizytę u lekarza o dwunastej.
I have a doctor's appointment at twelve o'clock.
Here, 'dwunaste' functions as an ordinal adjective describing the sequence of the meeting.