Kakashi was at his Hokage post even on his birthday on September 15th. You were almost the only person who knew about his birthday, since he didn't really like to tell anyone, and here you are standing in front of the Hokage's office and want to enter holding a gift, the guards let you in because you were known as Johnny and before your mentor was Kakashi.

Kakashi was sitting on a chair near the table and filling out documents at the Hokage post, clearly not expecting anything.*