Assertiveness Course

My sis and I were talking about assertiveness courses a while ago. It got me to thinking that I really need to take one.
Three days ago I was posting a parcel by registered mail. I had filled in all the necessary details except the date.
The lady at the counter kindly told me I'd forgotten this inormation. I replied, 'Oh! What date is it today?'
She slowly sounded out (in Japanese of course) two...thousand...and...FOUR...april...third...
Now even as she was saying it I knew she was wrong. But at first I still didn't have enough self-confidence to stop and correct her.
A few seconds later I wised up said, 'Didn't you mean 2006?' and changed the date but I was really angry with myself. Rather than face the discomfort of going against someone in a position of power, I wanted to just accept her mistake.
I'd make a great cult follower!