Hi Anonymous Monk,

You can use DateTime and friends to do this, comparing with subtract_datetime_absolute(), but you'll have to start with a delta in nanoseconds. The code below shows the difference in milliseconds as specified, including fractions.

use strict; use warnings; use feature 'say';
use DateTime::Format::Strptime;

my $parser = DateTime::Format::Strptime->new(
    pattern  => '%F %T.%6N',
    on_error => 'croak',
);

while ( <DATA> ) {
    chomp;
    my ( $t2, $t1 ) = split /, /;
    my $dt2 = $parser->parse_datetime( $t2 );
    my $dt1 = $parser->parse_datetime( $t1 );

    my $delta = $dt2->subtract_datetime_absolute( $dt1 );
    my $delta_ms = sprintf( '%.3f',
        $delta->in_units('nanoseconds') / 1_000_000,
    );

    say for ( $t2, $t1, "Differs by $delta_ms ms", '' );
}

__DATA__
2017-01-09 05:08:04.575804, 2017-01-09 05:08:04.319338
2017-01-09 05:08:17.093719, 2017-01-09 05:08:16.840292
2017-01-09 05:12:48.629407, 2017-01-09 05:12:48.372687
2017-01-09 05:13:25.315665, 2017-01-09 05:13:25.061834
2017-01-09 05:14:08.171994, 2017-01-09 05:14:07.916187
2017-01-09 05:17:03.182747, 2017-01-09 05:17:02.929309
2017-01-09 05:18:46.561289, 2017-01-09 05:18:46.305743
2017-01-09 05:19:09.772227, 2017-01-09 05:19:09.518975
[download]

perl 1179200.pl
2017-01-09 05:08:04.575804
2017-01-09 05:08:04.319338
Differs by 256.466 ms

2017-01-09 05:08:17.093719
2017-01-09 05:08:16.840292
Differs by 253.427 ms

2017-01-09 05:12:48.629407
2017-01-09 05:12:48.372687
Differs by 256.720 ms

2017-01-09 05:13:25.315665
2017-01-09 05:13:25.061834
Differs by 253.831 ms

2017-01-09 05:14:08.171994
2017-01-09 05:14:07.916187
Differs by 255.807 ms

2017-01-09 05:17:03.182747
2017-01-09 05:17:02.929309
Differs by 253.438 ms

2017-01-09 05:18:46.561289
2017-01-09 05:18:46.305743
Differs by 255.546 ms

2017-01-09 05:19:09.772227
2017-01-09 05:19:09.518975
Differs by 253.252 ms
[download]

Hope this helps!