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I saw National Treasure last night. It's silly, but fun if you can suspend enough disbelief to let it work. I noticed an interesting mistake having to do with the time on the clock at Independence Hall. The treasure-hunters wanted to be in a certain place at the same time of day as it was 200 years ago. And they remembered daylight savings time - but they forgot about time zones!

Time zones weren't invented until 1878. Before the invention of time zones, clocks reflected "sun time", varying by about a minute for every 12.5 miles you traveled west. Noon was when the sun was directly overhead; there wasn't a single noon for a huge region.

The invention of the telegraph necessitated knowing how many minutes apart various cities were and this information was printed in calendars at the time; my family has one such calendar. It was the creation of the railroads that necessitated standardized time zones - it was hard to make a decent railway schedule without them - and eventually these time zones were simplified into the set we have today.

Thus, we can be pretty sure the time that the treasure hunters looked for the shadow of the Independence Hall tower was wrong. The easiest way to find the proper time they should have been there would be to look at a local sundial and calculate the offset between the time that shows there and the time on your watch.

Maybe that means there's another treasure yet to be found!