If else statement with time

I have the following dataframe;

A  B  StartTime            EndTime
r  t  2018-06-11 00:00:00  2018-12-16 00:00:00
d  g  2018-06-22 05:30:00  2018-12-01 00:00:00      
r  g  2018-09-10 08:00:00  2023-01-01 00:00:00  
e  d  2018-09-26 23:00:00  2019-02-07 23:00:00  
.
.
.

y  h  2018-09-30 23:00:00

I need to add an extra column 'DayNight' based off whether or not the 'Start Time' starts at day or night.

early = 06:00:00
late =  17:59:00


if df['Start Date'] > early and df['Start Date'] < late:
    df['DayNight'] = 'Day'
else:
   df['DayNight'] = 'Night'

I'm trying to write an if statement but struggling on how to define a time without losing the dates.

Any help would be much appreciated.