How to compare values from 2 date sets in python?

I want to calculate 3 fields: start_count, new_count, end_count, completed_count where

count = no.of Item_ids  
start_count = previous day's end_count  
new_count = current day's count (item ids which are not present in previous day)  
completed_count = start_count - no.old item_ids
end_count = start_count + new_count - completed_count

df:

Date    Item_id    Count
    01/01/2020    100    1   
    01/01/2020    101    1   
    01/01/2020    100    1    
    02/01/2020    102    1    
    02/01/2020    101    1    
    02/01/2020    100    1    
    03/01/2020    101    1    
    03/01/2020    102    1    
    03/01/2020    103    1

df_result:

Date    Start_count    new_count    completed_count    end_count
01/01/2020    3    3    0    3
02/01/2020    3    1    1    4
03/01/2020    4    1    2    3

I tried:

df2 = pd.DataFrame()  
for d in dates:  
    if df.Item_id[d] in df.Item_id[d-1]:      
        df_calc['old'] = df.groupby('Date').agg({'Count':'sum'}).reset_index()  
        if not df_Item_id[d] in df.Item_id[d-1]:  
            df_calc['new'] = df.groupby('Date').agg({'Count':'sum'}).reset_index()  
    else:  
        df_calc['old'] = 0  
df2.append(df_calc)  

and got error: datetime.date(2020,1,1)