How do I convert dmesg timestamp to custom date format?

With the help of dr answer, I wrote a workaround that makes the conversion to put in your .bashrc. It won't break anything if you don't have any timestamp or already correct timestamps.

dmesg_with_human_timestamps () {
    $(type -P dmesg) "$@" | perl -w -e 'use strict;
        my ($uptime) = do { local @ARGV="/proc/uptime";<>}; ($uptime) = ($uptime =~ /^(\d+)\./);
        foreach my $line (<>) {
            printf( ($line=~/^\[\s*(\d+)\.\d+\](.+)/) ? ( "[%s]%s\n", scalar localtime(time - $uptime + $1), $2 ) : $line )
        }'
}
alias dmesg=dmesg_with_human_timestamps

Also, a good reading on the dmesg timestamp conversion logic & how to enable timestamps when there are none: https://supportcenter.checkpoint.com/supportcenter/portal?eventSubmit_doGoviewsolutiondetails=&solutionid=sk92677

Understanding dmesg timestamp is pretty simple: it is time in seconds since the kernel started. So, having time of startup (uptime), you can add up the seconds and show them in whatever format you like.

Or better, you could use the -T command line option of dmesg and parse the human readable format.

From the man page:

-T, --ctime
    Print human readable timestamps. The timestamp could be inaccurate!

    The time source used for the logs is not updated after system SUSPEND/RESUME.