The wrong month!

I was going to visit a customer in a town 5 hours from mine. I arrived after the long journey and I was standing outside the company´s entrance which was locked. I phoned the customer and said: - Hi, I am here now! He responded: - Today? Wasn´t it the same date next month? I replied: Eh, next month? The customer let me in and fortunately he had time for me. I later checked our email correspondence and he was right. I had written the wrong month! Ups....embarrassing...well better than one month too late..right!?