Parse date String to milliseconds and vice versa - perl

Question

I have following date in string format and I need to convert it to milliseconds using a Perl script. I have tried to convert it using DateTime::Format::Strptime and it returns 0 after convert to millisecond.
Date String : 01-13-15:14.16
#!/usr/bin/perl
use DateTime::Format::Strptime;
my $strp = DateTime::Format::Strptime->new(
pattern => '%m-%d-%y:%H.%M',
on_error => 'croak',
);
my $dt = $strp->parse_datetime("01-13-15:14.16");
print $dt->millisecond."\n";
How can I convert datetime to milliseconds

Answer 1

I think it's likely that what you want is not milliseconds but epoch, which is the number of seconds since January 1 1970. Your date-time format has neither seconds nor milliseconds, so both of these fields will be reported as zero, and if you really want milliseconds then you can simply multiply the epoch by 1000.
It looks like this
use strict;
use warnings;
use 5.010;
use DateTime::Format::Strptime;
my $strp = DateTime::Format::Strptime->new(
pattern => '%m-%d-%y:%H.%M',
on_error => 'croak',
);
my $dt = $strp->parse_datetime('01-13-15:14.16');
say $dt->epoch;
output
1421158560
However, the DateTime module is enormous and slow, and there is a better solution in the core module Time::Piece, which would look like this
use strict;
use warnings;
use 5.010;
use Time::Piece;
my $dt = Time::Piece->strptime('01-13-15:14.16', '%m-%d-%y:%H.%M');
say $dt->epoch;
output
1421158560

Answer 2

It is impossible to convert a date to milliseconds. A date is a point in time. Milliseconds measure a duration.
The only way your question makes sense is if we assume you mean "how many milliseconds are there between my given date/time and some other fixed date/time". Because of the way that Unix measures time, let's assume that the fixed date/time you're measuring from is the Unix Epoch (1970-01-01 00:00:00).
You get the number of seconds since the epoch from a DateTime object with the epoch() method.
say $dt->epoch;
To get the number of milliseconds, just multiply that by a thousand.
say 1000 * $dt->epoch;
Your current timestamp doesn't include milliseconds. If you change to parsing strings that include them, then you can add that value on too.
say $dt->milliseconds + (1000 * $dt->epoch);

Answer 3

Adding my comment as an answer:
If you want to convert the datetime value into seconds since the epoch, try one of these:
print sprintf("%f", $dt->epoch + ($dt->millisecond/1000));
print sprintf("%s.%s", $dt->epoch, $dt->millisecond);
(My original comment did not account for millisecond being an integer)
Similar for the values in milliseconds, rather than seconds.millis:
print sprintf("%d", ($dt->epoch * 1000) + $dt->millisecond);
print sprintf("%d%04d", $dt->epoch, $dt->millisecond);
Remember, that if you don't supply DateTime with a value containing milliseconds (or higher resolution), the value will simply be zero.