PYTHON

groupby year datetime pandas

data.groupby(data.date.dt.year)

pandas group by day

df.groupby([(df.Date.dt.month),(df.Date.dt.day)])

group by month and day pandas

max_temp = dfall.groupby([(dfall.Date.dt.month),(dfall.Date.dt.day)])['Data_Value'].max()

group by dateime pandas

df['date_minus_time'] = df["_id"].apply( lambda df : 
datetime.datetime(year=df.year, month=df.month, day=df.day))	
df.set_index(df["date_minus_time"],inplace=True)