Dear Marty, if my calculations are correct, you will receive this letter immediately after you saw the DeLorean struck by lightning.

7 seconds sound clip from the Back to the Future Part II movie soundboard.

You can hear this line at 01:40:01.421 in the Blu-ray version of the movie.

Quote context

[...]

- We've had that envelope in our possession for the past 70 years. It was given to us with the explicit instructions that it be delivered to a young man with your description answering to the name of Marty, at this exact location, minute, November 12, 1955.

- We have a little bet going as to whether this Marty would actually be here. Looks like I lost.

- Did you say 70 years?

- Yeah, 70 years, 2 months, 12 days to be exact.

- Here sign on line 6, please. Here you are.

- It's from the doc!

- Dear Marty, if my calculations are correct, you will receive this letter immediately after you saw the DeLorean struck by lightning.

- First, let me assure you that I'm alive and well. I've been living happily these past eight months in the year 1885. The lightning bolt...

- 1885! September, 1885!

- Wait, wait, kid! Wait a minute. What's this all about?

- He's alive! The doc's alive! He's in the Old West, but he's alive.

[...]