This is one of those classic quotes that, once you've heard it stays with you always because it is just so good to remember. It is one of my favourites and always reminds me to live in the present and not dwell on the past or worry about the future. It has been around for a long time in one form or another, as to it's true origins? I don't know, so if anyone knows where it comes from please share. It has been around and repeated so much that I wouldn't be surprised if it's true origins had been lost, as is the case with many poignant phrases:
"Yesterday is history, tomorrow is a mystery, but today is a gift. That is why it is called the present."