Hack 12 Understanding and Using Julian Dates
 
Get to know and use Julian Dates.
Date-based searching good! Date-based
searching with Julian dates annoying (for a human, anyway)!
The Julian date is the number of days that have passed since January
1, 4713 BC. Unlike Gregorian dates, which begin at midnight, Julian days begin
at noon, making them useful for astronomers.
A Julian date is just one number. It's not broken up
into month, day, and year. That makes it problematic for humans but
handy for computer programming, because to change dates, you simply
have to add and subtract from one number, and not worry about month
and year changes.
To use Google's date-range syntax in Perl,
you'll need a way to
convert the computer's local time to Julian. You can
use the module Time::JulianDay, which offers a variety of ways to
manipulate local time in Julian format. You can get the module and
more information at http://search.cpan.org/search?query=Time%3A%3AJulianDay.
Hacks that use the Julian date format and date-range searching pop up
throughout this book; start by learning more about using the
date-range syntax [Hack #11]. Also included are hacks for building recent
searches into a customized form [Hack #42], and date-range
searches with a client-side application [Hack #60].
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