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Dates and Times
The Silent Killers
Saturday, August 11, 12

Dates and Times Are Easy!



• 60 seconds in a minute, 60 minutes in an hour.



• 24 hours in a day, 365 days in a year, unless the year
is a multiple of 4.



• 24 hours in a day, 365 days in a year, unless the year
is a multiple of 4.

• Simple, right?


A reality check


A reality check

• There are not always 60 seconds in a minute,
sometimes more (leap seconds, joy!)


A reality check

• There are not always 60 seconds in a minute,
sometimes more (leap seconds, joy!)

• Not always 24 hours in a day: sometimes more,
sometimes less—consider DST


Pop Quiz!

• How many days were there in February 1900?


• How about February 2000?


• How about February 2000?
• Remember what you know about leap years.


• There were... 28 days in February 1900


• ... and 29 in February 2000


• ... and 29 in February 2000
• Why? Years that are divisible by 100 are not leap
years, whereas years divisible by 400 are


Why? How does that affect me? How do
I do it right?


I do it right?

• Variances in Earth’s rotation and axial tilt cause our
rotation and revolution to be inconsistent


I do it right?


• It means that there’s a lot of particulars and edge
cases for doing date/time math


I do it right?


• It means that there’s a lot of particulars and edge
cases for doing date/time math

• Currently, NYC observes DST. But the government
could change that. Certain parts of the US (Indiana,
Arizona) don't observe


If that wasn’t enough...

• Consider what you’d do if you were working for
Indiana Jones and he wanted you to convert dates
between the Mayan calendar and the Gregorian
calendar. Or, more likely, the Julian calendar (run cal
9 1752 and prepare to be surprised) and the
Gregorian calendar.


How do we handle this?


use DateTime;


use DateTime;

• DateTime knows about all these edge cases and
vagaries and handles them for you, intelligently


use DateTime;

• DateTime knows about all these edge cases and
vagaries and handles them for you, intelligently

• It throws exceptions for invalid data, so you can’t
construct something that shouldn’t exist


A Word about Timezones


• By default, DateTime objects use the “floating”
timezone


timezone

• This means that it doesn’t take into account any local
timezone issues like leap years


timezone

• This means that it doesn’t take into account any local
timezone issues like leap years

• It’s best to specify the timezone when creating a DT
object, convert to UTC for working with time/dates,
and then convert to the user’s local timezone for
display


my $dt = DateTime->now(time_zone =>
'America/New_York');

$dt->set_time_zone('UTC');
$dt->add(months => 3) # leap year!
# the next day...
$dt->set_time_zone('Asia/Taipei');


Getting Dat[ae] into DateTime



• Several constructors: new, now, from_epoch



• The new constructor takes parameters like hour,
minute, second, day, month, year




• from_epoch takes a named epoch argument




• from_epoch takes a named epoch argument
• All constructors take a time_zone named
argument


my $dt = DateTime->now(
time_zone => 'EST5EDT'
);
my $dt = DateTime->from_epoch(
epoch => 1234567890,
time_zone => 'America/New_York',
);
my $dt = DateTime->new(
minute => 24,
hour => 20,
year => 2012,
month => 3,
day => 1
);


Manipulating Dates


Manipulating Dates
• Arithmetic operations are overloaded


Manipulating Dates
• Subtracting two DateTime objects yields a
DateTime::Duration (the delta)


Manipulating Dates

• Adding two objects doesn't make sense; add
a ::Duration to get a DateTime object


Manipulating Dates

• Adding two objects doesn't make sense; add
a ::Duration to get a DateTime object

• See also add, subtract methods (take same args as
new)


my $later = $dt->clone->add(days => 2);
my $duration = $later - $dt;
my $later_also = $dt + $duration;


Getting Dat[ae] Out


Getting Dat[ae] Out

• DateTime overloads the " operator; this produces an
ISO8601-formatted string. There are lots of other
formatters on CPAN, including one for MySQL types.


Getting Dat[ae] Out


• There's a strftime method for very flexible
formatting


Getting Dat[ae] Out


• There's a strftime method for very flexible
formatting

• Other simpler methods, like ymd, as well as varied
accessors


my $announcement = "It is now $dt";
my $proper_date = $dt->strftime(
"%Y-%m-%d"
);
my $identical = $dt->ymd;


Questions?


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