The purpose of this library was to replace our dependence on Unix epoch
time, which, being limited to a range of about 1970 to 2030, is inadequate
for our purposes (we have data as old as 1870). This date library
effectively handles dates from A.D. 1000 to infinity, and would probably
work all the way back to 0 (ignoring, of course, the switch-over to the
Gregorian calendar). The useful features of Unix epoch time (ease of date
difference calculation and date comparison, strict ordering) are preserved,
and elements such as human-legibility are added. The library handles
fractional seconds and some date/time manipulations used for the Global
Positioning Satellite system.

The operators +/-, <=>, cmp, stringify are overloaded. Addition handles
seconds and fractions of seconds, subtraction handles seconds or date
differences, compares work, and stringification returns the a
representation of the date.

The US Geological Survey (USGS) likes midnight to be 24:00:00 of the
previous day, not 00:00:00 of the day people expect. If
$DateTime::Precise::USGSMidnight is set, dprintf will always print midnight
as 24:00:00 and the date returned from dprintf will have the previous day's
date. Regardless, time is always stored internally as 00:00:00.