day HIGH/LOW studies on 1-min S&P futures data.

[scidam]

What are column types (date and time)?

If they are of DateTime dtype (datetime64[ns]), you can do this using groupby, e.g.

df.groupby('date')['high'].max() # get max values of 'high' column for each day
df.groupby('date')['high'].min() # get min values of 'high' column for each day

df.groupby([df.date, df.time.dt.hour])['high'].max() # get max values of 'high' column for each hour (during the day)