Stack Exchange network consists of 175 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.
Please arrive at the venue not earlier than 15 minutes before the commencement of the interview.
In plain English, do not arrive more than 15 minutes early for your interview.
"Not earlier than" means later, i.e. you can arrive 15 minutes before the interview or later. Five or ten minutes before the interview, for instance.
"Not earlier than X" means "after X".
"Not later than X" means "before X".