I am trying to create an array that converts minutes, seconds, and milliseconds to seconds and adds them consecutively. 

I have a df with a timestamp as follows:

```
        Time        Temp.
0   14:12:58:8230    135
1   14:12:59:0190    185
2   14:12:59:2150    138

```

I do the following to retrieve the data and access all the Timestamp column with `datetime.strptime` and make the array. But, when running it, I seem to have a `KeyError : 0`. 

```
elapsed_x = []
tmp_1 = []

for i in range (0, len(df)): 
    tmp = datetime.strptime(df.Time[i], '%H:%M:%S:%f') 
    tmp_ms = tmp.microsecond
    tmp_s = tmp.second
    tmp_min  = tmp.minute
    tmp_1.append(tmp_min*60 + tmp_s + tmp_ms/1000000)

for i in range (0, len(df)):     
    elapsed_x.append(tmp_1[i]-tmp_1[0])

```

What am I doing wrong?