Slicing Datetime Function: Python

I have a dateframe with the following column:

0    2019-06-30 17:31:43
1    2019-06-30 07:07:41
2    2019-06-30 17:11:46
3    2019-06-30 12:13:50
4    2019-06-30 12:13:55
5    2019-06-30 03:01:53
6    2019-06-30 07:22:02
7    2019-06-30 18:12:47
8    2019-06-30 21:38:19
9    2019-06-30 03:01:58
10   2019-06-30 10:06:16
11   2019-06-30 03:46:43
12   2019-06-30 00:44:24
13   2019-06-30 00:44:29
14   2019-06-30 00:44:32
15   2019-06-30 00:44:33
16   2019-06-30 19:32:09
17   2019-06-30 09:09:42
18   2019-06-30 11:52:10
19   2019-06-30 18:30:00
20   2019-06-30 01:26:56
21   2019-06-30 19:29:02
22   2019-06-30 11:54:39
23   2019-06-30 16:54:47
24   2019-06-30 06:00:49
25   2019-06-30 18:51:17
26   2019-06-30 05:54:55
27   2019-06-30 19:22:43
28   2019-06-29 23:06:12
29   2019-06-30 04:26:07

What I'm after is a new column that has the time of day [Morning, Afternoon, Evening].

The following line returns a Boolean

test['visitStartTime_aest'].dt.strftime('%H:%M:%S').between('00:00:00','12:00:00') 

This line returns a filtered dataframe and changes the value to morning [which is what I want]

test[test['visitStartTime_aest'].dt.strftime('%H:%M:%S').between('00:00:00','12:00:00')] = 'Morning'

This works, but now that column has mixed dtypes making it hard to transform the remaining timestamps as some are now str and others are datetime64[ns]

I have tried the following with no luck:

def time_of_day(df, date_col):
  df[date_col] = df[df[date_col].dt.strftime('%H:%M:%S').between('00:00:00','12:00:00')] = 'Morning'
  df[date_col] = df[df[date_col].dt.strftime('%H:%M:%S').between('12:00:01','18:00:00')] = 'Afternoon'
  df[date_col] = df[df[date_col].dt.strftime('%H:%M:%S').between('18:00:01','23:59:59')] = 'Evening'
  return df

This executes the first row perfectly but the following rows suffer the same fate as above [mixed dtypes]

I have also tried this, but with no luck:

def time_of_day(df):
  if df['visitStartTime_aest'].dt.strftime('%H:%M:%S').between('00:00:00','12:00:00'):
    return 'Morning'
  elif df['visitStartTime_aest'].dt.strftime('%H:%M:%S').between('12:00:01','18:00:00'):
    return 'Afternoon'
  else:
    return 'Evening'

test.apply(time_of_day, axis=1)

Any idea as to what I'm missing? Or any guidance on how to execute?

Thanks!