Why do we have 2 date fields in the wp_posts? What's the purpose of the post_date_gmt? When does storing the greenwich time get handy?
Some countries use a Daylight Saving Time (DST):
Sometimes there is no 2 am. Sometimes you get 2 am two times. To avoid cron jobs running twice or not at all WP needs the GMT (or more precise: UTC). UNIX time stamps cannot be used because they start in 1970 … and there is still the Year 2038 problem.
If you want to write an imaginary diary of Gaius Julius Caesar or James T. Kirk … you will run into problems with the current system too, but they may be solvable at least. :)