I made a mistake on the promised time.

Today, I had an appointment with my friend at his home.
I thought that our appointment time was (is at) 4pm and I bided my time (spent) drinking a cup of coffee alone in the café.
Suddenly, my friend sent me a text message asking where I was  (of where you are.)  We had an appointment at 3pm,he wrote.   I felt that I made a mistake over the time and answered that I had forgotten our appointment. Fortunately, my friend said that we could (would) meet on Saturday.
I    said I was really sorry/apologised    to (for) my friend.  I had to write a diary note to remember this next  important meeting.  ( next time.)