Alacrity

Engineer-nerd alert:

Watchmaker With Time to Lose - because of the difficulty of constant conversions to "Mars time", NASA enlisted the help of a watchmaker to create a watch that runs 39 minutes slow.

Seems like it would be easy to just tell a watch to run slower... but obviously not. As always, you can get more detailed info on how watches work from How Stuff Works.

(link courtesy of metafilter)