change item in column based on the item before[solved]

[Larz60+]

Your code can be greatly simplified.
The following opens the input spreadsheet you supplied, and writes it back out as a csv.
It does nothing to modify the data, you will have to add that.
The print statement only shows what was read and should be removed when statisfied

import os
import pandas as pd

# make sure starting directory set
os.chdir(os.path.abspath(os.path.dirname(__file__)))
n = 2
df = pd.read_excel("newfile1.xlsx", dtype=str)
print(df)
pd.to_csv('newfile2.csv')

Output:
     datetime,intensity
0   19790111-1007,3.333
1   19790111-1007,3.333
2   19790111-1007,3.333
3   19790111-1007,0.556
4   19790111-1007,0.556
5   19790111-1007,0.556
6   19790111-1007,0.556
7   19790111-1007,0.556
8   19790111-1007,0.556
9   19790111-1007,3.333
10  19790111-1007,0.370
11  19790111-1007,0.370
12  19790111-1007,0.370
13  19790111-1007,0.370
14  19790111-1007,0.370
15  19790111-1007,0.370
16  19790111-1007,0.370
17  19790111-1007,0.370
18  19790111-1007,0.370
19  19790111-1007,0.476
20  19790111-1007,0.476
21  19790111-1007,0.476
22  19790111-1007,0.476
23  19790111-1007,0.476
24  19790111-1007,0.476
25  19790111-1007,0.476