Perl date calculation and formatting

[Anonymous Monk]

For example I read a file listed with dates:

in.txt:
Mon Mar 30 10:00:00 2013
Sun Mar 29 10:00:00 2013
Sat Mar 28 10:00:00 2013

and I want to subtract 30 days from each date and save them to out.txt in exactly this same format Of course I can read the file and tokenize them, but I don't know how to subtract date(and with same format).. what is the proper module for this job?

[pme]

You may find these modules useful too: Date::Calc, Date::Manip.

[i5513]

If you are on linux, you can always play with date command:

date -d 'Mon Mar 30 10:00:00 2013 + 30 day'

with perl, I would try DateTime
Regards,

[Anonymous Monk]

Okay thanks!!

[ambrus]

I recommend the Date-Manip module for most date calculations in perl.

Sadly, this is one of those date formats that Date::Manip does not currently automatically detect, so you have to use an explicit format string to parse it. If you try to parse the dates with an explicit format string, you will find that they are invalid, because Mar 30, 2013 was a Saturday, but the first line says "Mon" instead of "Sat". The other two dates are wrong as well. So let's parse the dates but ignore the day of week given in the input, and print the correct date 30 days before that.

use warnings; use Date::Manip::Date 6.30; 
my$base = Date::Manip::Date->new; $base->config("setdate" => "now,utc"
+);
my $delta = $base->new_delta("-30 days"); 
open my$infile, "<", "in.txt" or die; 
while (<$infile>) { 
    my$cur = $base->new; 
    my $parse_err = $cur->parse_format(q"\w+ %b %d %H:%M:%S %Y", $_); 
    if ($parse_err) { 
        1; # input line does not contain a valid date, just skip it
    } else { 
        my$mod = $cur->calc($delta); 
        print $mod->printf("%a %b %d %H:%M:%S %Y"); 
    } 
    print "\n"; 
}
__END__

The output you get is this:

Thu Feb 28 10:00:00 2013
Wed Feb 27 10:00:00 2013
Tue Feb 26 10:00:00 2013

See also the related questions: Find 30 days from today's date and Re: Can Date::Manip parse a unix timestamp?.

[hippo]

To my mind the FAQ is both clear and comprehensive on this. Did you have any trouble understanding it?