If you're editing a file in Vim you can rewind time to a previous version of the file with the earlier command. You can go backwards (or forwards) in the history of a file based on a given time measurement. This can be helpful if you mess up your file and just want to rollback to a previous version.

:earlier 5m

or

:later 5m

Reddit had some interesting discussion on what you can do with this feature.

Note: Alternately you can use :e! to reload the file from disk if you haven't saved since your mess up.

Perl function to return a human readable string for a time duration in seconds.

my $str = human_time_diff(320);  # "5 minutes"
my $str = human_time_diff(3700); # "1 hour"

sub human_time_diff {
    my $seconds = int(shift());
    my $num     = 0;
    my $unit    = "";
    my $ret     = "";

    if ($seconds < 120) {
        $ret = "just now";
    } elsif ($seconds < 3600) {
        $num  = int($seconds / 60);
        $unit = "minute";
    } elsif ($seconds < 86400) {
        $num  = int($seconds / 3600);
        $unit = "hour";
    } elsif ($seconds < 86400 * 30) {
        $num  = int($seconds / 86400);
        $unit = "day";
    } elsif ($seconds < (86400 * 365)) {
        $num  = int($seconds / (86400 * 30));
        $unit = "month";
    } else {
        $num  = int($seconds / (86400 * 365));
        $unit = "year";
    }

    if ($num > 1) { $unit .= "s"; }
    if ($unit) { $ret = "$num $unit"; }

    return $ret;
}

See also: PHP version

function human_time_diff(int $seconds) {
    $num  = 0;
    $unit = "";

    if ($seconds < 300) {
        $ret = "just now";
    } elseif ($seconds < 3600) {
        $num  = intval($seconds / 60);
        $unit = "minute";
    } elseif ($seconds < 86400) {
        $num  = intval($seconds / 3600);
        $unit = "hour";
    } elseif ($seconds < 86400 * 30) {
        $num  = intval($seconds / 86400);
        $unit = "day";
    } elseif ($seconds < (86400 * 365)) {
        $num  = intval($seconds / (86400 * 30));
        $unit = "month";
    } else {
        $num  = intval($seconds / (86400 * 365));
        $unit = "year";
    }

    if ($num > 1) {
        $unit .= "s";
    }

    if ($unit) {
        $ret = "$num $unit";
    }

    return $ret;
}

See also: Perl version

Inspired by Qlock I designed an HTML5 version to show you the time using words.

Linux has a common date/time format used in logs that looks like May 4 01:04:16. Often I will need to parse that into a unixtime so I wrote a function to do it so I won't have to reinvent the wheel next time:

use Time::Piece;

my $epoch = linux_timestr("May  4 01:04:16");

sub linux_timestr {
    my $time_str = shift();
    # Since this string type doesn't include the year we append the current
    # year to make the calculations correct. Otherwise we get 1970
    my $year     = (localtime())[5] + 1900;
    $time_str   .= " $year";

    my $format = "%b %d %H:%M:%S %Y";
    my $x      = localtime->strptime($time_str, $format);

    return $x->epoch();
}

Other common formats are cdate and ISO 8601

# cdate
my $x = localtime->strptime("Sat May  8 21:24:31 2021", "%a %b %d %H:%M:%S %Y");
# ISO 8601
my $y = localtime->strptime("2000-02-29T12:34:56", "%Y-%m-%dT%H:%M:%S");

Perl has a great core module for dealing with dates and times: Time::Piece.

use Time::Piece;

my $t = localtime();

my $unixtime = $t->epoch(); # Unixtime
my $human    = $t->cdate(); # Human readable

# Format a date/time for output
print $t->strftime("%Y-%M-%d") . "\n";

# Convert a specific format to a date/time object
my $bd = localtime->strptime("1985-02-14", "%Y-%M-%d");
print "You were born on a " . $bd->fullday . " in " . $bd->year . "\n";

# Date/Time addition
print "In one hour it will be: " . (localtime() + 3600)->hms . "\n";

It works by overriding the built in localtime() and gmtime() functions and giving them an object oriented interface. I highly recommend looking at it if you have to deal with dates and times.

Some quick perl to return the number of seconds in the current day since midnight.

sub midnight_seconds {
   my @time = localtime();
   my $secs = ($time[2] * 3600) + ($time[1] * 60) + $time[0];

   return $secs;
}

The same code just written in PHP.

function midnight_seconds() {
   $secs = (date("G") * 3600) + (date("i") * 60) + date("s");
   return $secs;
}