Это даст вам df с datetime64 [ns] и столбцом типа dtype для ваших данных:
import pandas as pd
df = pd.read_csv('hm.txt', sep=r"[ ]{2,}", engine='python', header=None, names=['pre'])
df['pre_1'] = df['pre'].astype(str).str.replace('00', '')
df['datetime_dtype'] = pd.to_datetime(df['pre_1'], format='%H', exact=False)
df['str_dtype'] = df['datetime_dtype'].astype(str).str[11:16]
print(df.head(5))
pre datetime_dtype str_dtype
0 1 1900-01-01 01:00:00 01:00
1 2 1900-01-01 02:00:00 02:00
2 3 1900-01-01 03:00:00 03:00
3 4 1900-01-01 04:00:00 04:00
4 5 1900-01-01 05:00:00 05:00
print(df.dtypes)
pre object
datetime_dtype datetime64[ns]
str_dtype object
dtype: object