#!perl

use 5.10.0;
use strict;
use warnings;

my %merged = ();
my $index = 0;

map {
    open my $file, '<', $_ or die $!;

    map {
        $merged{$_->[0]} //= [ qw{null} x 3 ];
        $merged{$_->[0]}[$index] = $_->[1];
    }
    map {
        [ m{ \A ( \S+ \s \S+ \s \S+ \s \S+ ) \s ( \S+ ) \z }msx ]
    }
    map {
        chomp; $_
    } (<$file>);

    close $file;

    ++$index;
} qw{gravity magnetics bathymetry};

say join(' ', $_, @{$merged{$_}}) for sort keys %merged;


##</code><code>##

ken@Miranda ~/c/_/tmp
$ cat gravity
2010-10-01 00:00:03 lat1 long1 grav1
2010-10-02 00:00:05 lat2 long2 grav2
2010-10-03 00:00:07 lat3 long3 grav3

ken@Miranda ~/c/_/tmp
$ cat magnetics
2010-10-02 00:00:05 lat2 long2 mag1
2010-10-03 00:00:07 lat3 long3 mag2
2010-10-04 00:00:09 lat4 long4 mag3

ken@Miranda ~/c/_/tmp
$ cat bathymetry
2010-10-03 00:00:07 lat3 long3 bath1
2010-10-04 00:00:09 lat4 long4 bath2
2010-10-05 00:00:01 lat3 long3 bath3

##</code><code>##

ken@Miranda ~/c/_/tmp
$ geo_file_join.pl
2010-10-01 00:00:03 lat1 long1 grav1 null null
2010-10-02 00:00:05 lat2 long2 grav2 mag1 null
2010-10-03 00:00:07 lat3 long3 grav3 mag2 bath1
2010-10-04 00:00:09 lat4 long4 null mag3 bath2
2010-10-05 00:00:01 lat3 long3 null null bath3