Timetravel

What's wrong with this story:

I come to you and i say: "I can time travel. To prove this, im going to write in this letter, something that i will plant in the past, so you can find it in the present. You're not allowed to open the letter until i am back." - I proceed to the other room and a huge light fills the chamber, i come back. "Ok, you can open the letter".

You open the letter and... it's a drawing of stone henge.

Am I a charlatan, or did I plant the stones there?