Answer:
<em>Son las siete</em> refers to the current time while <em>a las siete</em> refers to a future event that will occur during that time.
The first is used wen telling the current time, while the latter is used to an established time that something is set to occur.
Explanation:
Son las siete de la noche
it is (currently) 7 at night
A las siete ma voy a duchar
At 7 (am or pm depending on the context) I am going to shower
