How To Sum Numbers Of Next 7 Days

[Developer_2018]

I would like to sum up the seconds that each employee spoke on the phone over the next 7 days.

ID; Person_nr; Name; Datum; Datum_plus7; Call_As_Sekunde
3469; 1111; Musterman, Muster; 19.01.2020; 25.01.2020; 0
3470; 1111; Musterman, Muster; 20.01.2020; 26.01.2020; 30600
3471; 1111; Musterman, Muster; 21.01.2020; 27.01.2020; 30600
3472; 1111; Musterman, Muster; 22.01.2020; 28.01.2020; 30600
3473; 1111; Musterman, Muster; 23.01.2020; 29.01.2020; 30600
3474; 1111; Musterman, Muster; 24.01.2020; 30.01.2020; 18000
3475; 1111; Musterman, Muster; 25.01.2020; 31.01.2020; 0
3476; 1111; Musterman, Muster; 26.01.2020; 01.02.2020; 0
3477; 1111; Musterman, Muster; 27.01.2020; 02.02.2020; 30600
3478; 1111; Musterman, Muster; 28.01.2020; 03.02.2020; 30600
3479; 1111; Musterman, Muster; 29.01.2020; 04.02.2020; 30600
3480; 1111; Musterman, Muster; 30.01.2020; 05.02.2020; 30600
3481; 1111; Musterman, Muster; 31.01.2020; 06.02.2020; 18000
16698;1111; Musterman, Muster; 01.02.2020; 07.02.2020; 0
16699;1111; Musterman, Muster; 02.02.2020; 08.02.2020; 0
16700;1111; Musterman, Muster; 03.02.2020; 09.02.2020; 30600
16701;1111; Musterman, Muster; 04.02.2020; 10.02.2020; 30600
16702;1111; Musterman, Muster; 05.02.2020; 11.02.2020; 30600
16703;1111; Musterman, Muster; 06.02.2020; 12.02.2020; 30600
16704;1111; Musterman, Muster; 07.02.2020; 13.02.2020; 18000
16705;1111; Musterman, Muster; 08.02.2020; 14.02.2020; 0
16706;1111; Musterman, Muster; 09.02.2020; 15.02.2020; 0
I wrote this code:

i=0     
j=0     
Summe=0       
total_7days=[]
#Current Day in row                                                            
for i in range(0,24): 
      # the next 7 days                                                                                                                                                                              
      for j in range(0,7):
       if (df['Datum'][i+j] <= df['Datum_plus7'][i] and df['Personal_nr'][i+j] == df['Personal_nr'][i]  ):
           Summe +=df['Call_AS_Sekunde'][j]
           print(str(df['Personal_nr'][i])+ '---'+ str(df['Personal_nr'][i+j])+ '---' + df['Datum'][i+j] +'---' +df['Datum_plus7'][i]+'---'+ str(df['call_As_Sekunde'][i+j]))
   total_7days.append(Summe)
   Summe=0
   print(total_7days[i])

Output:
Person_nr      Datum        Datum_7_Days  Call_As_Sekunde 
...                                 
1111---1111---25.01.2020---31.01.2020---  0                                                                           
1111---1111---26.01.2020---31.01.2020---  0                                                                                                                                   
1111---1111---27.01.2020---31.01.2020--- 30600                                                                   
1111---1111---28.01.2020---31.01.2020--- 30600                                                                        
1111---1111---29.01.2020---31.01.2020--- 30600                                                                            
1111---1111---30.01.2020---31.01.2020--- 30600                                                                               
1111---1111---31.01.2020---31.01.2020--- 18000                                                                                  
                  SUM of last seven days=140400  
                                                                                               
1111---1111---01.02.2020---01.02.2020---   0
                  SUM of last seven days=30600 

Error:
The code works correctly until 25.01.2020. From 26.01.2020 on it doesn't work properly because the 7th day is in the next month. I mean from 26.01.2020 to 31.01.2020 there is no calculation, because the next 7 days are in the next month! 
[b]26.01.2020 to 01.02.2020 ??? 
   27.01.2020 to 02.02.2020 ???
.. 31.01.2020 to 06.02.2020 ???[/b] 

How can I solve this problem?

Thanks a lot for your help

Best regards