Duration in seconds the session lasts
If you want to get the time duration between the start and end in seconds, you can try this : 

data %&gt;% group_by(SessionID) %&gt;% 
summarise(start_time = min(Timestamp), end_time =max(Timestamp)) %&gt;% # using your code until here
mutate(`session end_time in seconds` = as.numeric(difftime(end_time, start_time, units = "secs"))) # gives the time interval in seconds