Используйте Series.dt.floor
по дням, а затем добавьте 12 hour
s:
df = pd.DataFrame({'date':pd.to_datetime(['2013-10-01 14:32:00','2013-10-01 04:32:00'])})
df['date1'] = df['date'].dt.floor('D') + pd.Timedelta(12, unit='h')
Или:
df['date1'] = df['date'].dt.floor('D') + pd.offsets.DateOffset(hours=12)
print (df)
date date1
0 2013-10-01 14:32:00 2013-10-01 12:00:00
1 2013-10-01 04:32:00 2013-10-01 12:00:00