Computers have a simple way of keeping time: generally, they
count seconds. Real Computers -- those running Unix-like
operating systems -- count the number of elapsed seconds since
midnight January 1 1970, UTC. As I write this, it's about
1299218806 in this system. That number, often called "Unix time"
or a "time_t", is the same in Baltimore, London, or Tokyo.
Simple. But as experienced by us biological organisms, trying to
keep our activities somewhat in sync with the diurnal light-dark
cycle and the lengthening and shortening of the day over the
seasons, time is much trickier. Most of us only have to think
about that when we change the clocks in the spring and fall, or
when traveling to a new time zone. But on the internet, a
computer in Baltimore might be connected to one in London,
several timezones away; and those computers might be analyzing
data containing timestamps -- not time_ts like 1299218806, but
human-meaningful times like March 4 2011, 1:06:46 am -- from
months ago, when daylight savings time w