#!perl -nlwa
# log files with lines prefixed by unix time.
# subtract the first time from all the rest for time elapsed
unless(defined($prev)){
    die unless /^(\S+):/;
    $prev=$1;
    print;
}else {
    die unless ($t,$rest)=/^(\S+)(:.*)/;
    printf "%.3f%s\n",($t-$prev)/60,$rest;
}