There is no rule in connection to this question. Simply, run whenever you like. It is up to you what time you will choose and what time of the day you feel the best and ready to exercise. 

However, the body feels and reacts different in different times of the day. It is important whether you are hungry or with full stomach, tired or not, is your energy level low or high and much more factors. 

If you do your training in the morning you already did a big thing and it is off your mind for the rest of your day. You will wake your body, energize it and feel good all day long. However, it is not the easiest thing to the because all the body functions are at theirs lowest levels. But the fact that it is not the easiest thing to do may result in a good way because you become both mentally and physically stronger.

If you have the time it is better to leave the body stabilize its functions in the early morning and go for a run a bit later, before your lunchtime.

Same as morning running, the running in the mid of the day may not be the perfect choice because the body functions are not at their best. You may feel hungry or tired from work and you need little bit of rest before you start your training. It is not forbidden to run this times of the day but if you plan running at your best pace and make hard training it is better to save it for later.

It is said that the best time for running is in the afternoon and that most physical activities are best performed that time of the day.Every study about this showed that all the body functions are at theirs highest levels at this period of the day. The only thing that seems negative is that the motivation might be at its lower because of the all day spent in the office or any other occupations you had that day. 

The best thing is to try running in different parts of the days and see how you feel, when you feel strongest and most motivated and what is the impact of the training in that particular part of the day over the rest of the day.