Should I call my manager ?

I started a new job two weeks ago. I come to wrok at 8:30 am and need to leave at 5:30 to pick up my daugther before 6:00pm.  Since most of my coworker including my manager come to work around 10am, so they stay late around 6:30 to 7:00.

My manager is a nice person, he gave me training on my work. He usually starts meeting with me at 4:00pm, it is always over 5 to finish the session. Today he seems have more stuff to go over with me. I have to interrupt him by saying "I am sorry, I have to go now". I felt uncomfortable now by not giving him the reason why I have to leave.  Should I call him to explain since he looked surprised when I interrupted his talk? What should I do for next week, if same situation comes up? Without explaination again? 

I like the job, and can learn a lot of things. Please help, I do not want to offend him. I am waiting online for your input.