in reply to Re^2: What's "Better" DateTime or localtime?
in thread What's "Better" DateTime or localtime?

One thing to watch out for is daylight savings conversions when adding/subtracting days. E.g., this may not work as expected, depending on what day and what time of day it is run (just as if using the builtin localtime):
use Time::Piece qw(localtime);
use Time::Seconds qw(ONE_DAY);

my $t = localtime;
my $yesterday = $t - ONE_DAY();
my $tomorrow  = $t + ONE_DAY();
print "Today: ", $t->ymd(),"\n";
print "Yesterday: ", $yesterday->ymd(),"\n";
print "Tomorrow: ", $tomorrow->ymd(),"\n";

[download]
I have found this sort of mistake in someone's code before (though the code did not use Time::Piece, just plain localtime). One way to be sure of adding or subtracting the correct number of days would be to truncate to the beginning of the day, then add an extra half day, or subtract a half-day less:
my $t = localtime->strptime(localtime->ymd(), '%Y-%m-%d');
my $yesterday = $t - ( 0.5 * ONE_DAY() );
my $tomorrow  = $t + ( 1.5 * ONE_DAY() );
print "Today: ", $t->ymd(),"\n";
print "Yesterday: ", $yesterday->ymd(),"\n";
print "Tomorrow: ", $tomorrow->ymd(),"\n";

[download]
One advantage of DateTime is that it does make it a little easier to get this correct. E.g. in my timezone:
my $t = localtime->strptime('2013-03-11', '%Y-%m-%d');
my $yesterday = $t - ONE_DAY();
my $tomorrow  = $t + ONE_DAY();
print "Today: ", $t->ymd(),"\n";
print "Yesterday: ", $yesterday->ymd(),"\n";
print "Tomorrow: ", $tomorrow->ymd(),"\n";

#Prints:
Today: 2013-03-11
Yesterday: 2013-03-09
Tomorrow: 2013-03-12

[download]

Replies are listed 'Best First'.
Re^4: What's "Better" DateTime or localtime?
by mmartin (Monk) on Jul 18, 2013 at 14:51 UTC
    Hey runrig, thanks for the reply!

    Cool, good info there, thanks!

    I don't know if its the "best" way to go, but I was using the dates I created using the "Time::Piece" Module, then
    I use the "Date::Manip" module to calculate a future date/time (*or past date if you needed to). The cool thing about
    the Date::manip Module is that you can use human relate-able strings to calculate the date.

    For example, before I added the "Date::Manip" Module, and currently still, I have a Command Line Option that the user inputs when
    executing. Where the value of the argument is in the form of "a number followed by a time-unit". So for instance the
    user could enter any of the following:
            --end-in="1hour"
            --end-in="2 hours"
            --end-in="3 hrs"
            --end-in="20 min"
            --end-in="30minutes"
            --end-in="1day"
            etc...............

    Then after the user executes and after some error checking of the users input, I take the option the user entered, like "30 mins"
    and then use that in the DateCalc Function, which is part of the Date::Manip Module. For the error checking I split the users
    input into a number and a "time_unit", so "30mins" would get split into "30" and "mins", then I check each one.

    So I would then take the current_date calculated by "Time::Piece qw(localtime)" and use that as the first argument to the "DateCalc"
    Function, like so:
       ## Create the 'date manipulation' string for the DateCalc Function:
   #        *The user's input is called $end_timeSpan...
   my $date_manip = "in $end_timeSpan"

   # For this example lets say:
   #       $current_time == "07/17/2013 10:30:00"

   ### Calculate the future date from the $current_date:
   #      *Output from function is in the form:
   #           "2013071811:00:00" --> YYYYMMDDHH:MM:SS
   my $future_date = DateCalc($current_date,$date_manip)

   # After running the above & capturing each piece from DateCalc, the
   # $future_date == "07/17/2013 11:00:00", which would be 30 mins 
   # from the $current_date.

    [download]

    So that's basically what I'm doing with the Date stuff. There is some other stuff I do with the Date and Time but that above is
    the most important part.

    Thanks again for all the suggestions and info.


    Thanks Again,
    Matt


[reply]
[d/l]

In Section Seekers of Perl Wisdom