Local time can be very important in these circumstances and probably several else. There are a lot of software programs and browser add-ons that display multiple time zones to the user. Firefox users for example can install the excellent computer alarm clock and timer Simple Timer which displays a clock in the Firefox status bar.

Localti.me is an online service. The major benefit of it is the fact that it can be used to quickly look up the local time, local data and local weather without installing and software or plugin. All it takes is to enter a country, state or region to get the requested information. The service will display suggestions as soon as the user begins typing in the name.

The results page will list the local time, local data and local weather along with other information about the selected location. These other information include maps (taken from Virtual Earth), Wikipedia information, the latest headlines, traveling information and pictures taken from Flickr and Panoramio.

Localti.me (via Make use of) is a nice to have service for quickly looking up the local time, date or weather of a location in the world.

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Naturally the second reason is far more important than the first. But either way, you might very well be driven to keep your machine on the exact time. To that end you can either constantly monitor the time on your machine (manually changing it when necessary) or you can set up NTP to handle your time corrections for you.

Installing and configuring NTP is actually quite easy. It will, however, require the use of the command line. With that in mind, lets’ get busy.

Installing

You could open up your Add/Remove Software utility, do a search for “ntp” (no quotes) and install the matching results. Or you can open up a terminal window and install ntp with one of the following commands (remember, you will need either root access or sudo):

• apt-get install ntp
• yum install ntp
  
• urpmi ntp

One of the above commands will install the ntp daemon and the configuration file /etc/ntp.conf.

Before you fire up the daemon you will need to take a look at the /etc/ntp.conf to make sure you have the ntp servers you want to use configured.

Configuring

Most likely your NTP installation will already have a configuration file that is ready to go. My Debian-based NTP install had such a .conf file. All I had to do was start the daemon. But you might have special needs or your install might not have been as complete.

The section of the ntp.conf that you will want to take a look at is the server section. Each line in this section is set up like so:

server IP_ADDRESS OPTION(S)

Where IP_ADDRESS is the actual address of the server you want to use and OPTION(S) is/are the option(s) you want to use.

There are two options that you might want to use for every server. These are:

iburst: This option is used when the configured server is unreachable. When your machine can not contact its NTP server it will send out bursts of eight packets (instead of just one).

dynamic: This option allows a server to be configured even if the server is not reachable during configuration. This option assumes that at some point the server will be reachable.

If you look at my /etc/ntp.conf file you will see the following in the server section:

server 0.debian.pool.ntp.org iburst dynamic
server 1.debian.pool.ntp.org iburst dynamic
server 2.debian.pool.ntp.org iburst dynamic
server 3.debian.pool.ntp.org iburst dynamic

If you are not sure what servers to use visit the official NTP organization NTP.org to find a list of trusted servers.

Starting the Daemon

Once you have everything up and running go back to the root terminal and issue the command:

/etc/init.d/ntp start

or

/etc/rc.d/init.d/ntp start

which will start the ntp daemon. NTP will now slowly start to adjust the time on your server. But don’t worry, it makes the adjustment slowly. First the daemon waits for at least ten packets of information before trusting a source.

Now test to make sure your installation is working by issuing the following command:

ntpq -p

which should give you a listing like:
remote refid st t when poll reach delay offset jitter =========================================================
+point2.adamants 64.202.112.75 2 u 44 64 377 75.955 -7.045 2.992
*station.mars.or 209.81.9.7 2 u 10 64 377 75.477 -1.144 0.977
-www.broadbandja 64.34.180.101 3 u 56 64 377 72.764 3.766 0.977
+pxe.lax-noc.com 209.81.9.7 2 u 25 64 377 65.686 -7.753 2.539

If you see zeros for the values you know ntp is not connecting. As you can see above my setup is connecting.

Final thoughts

NTP is an easy way to ensure your servers or your desktop is always using the correct time. In mission-critical machines, this can be quite important. Employing NTP will give you the security of knowing your machine’s right “on time”.

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Echoing the time() function will give you the amount of time passed since the Unix Epoch in seconds. Say what? Epoch means a point in time chosen as the start of a period or an era and thus the Unix Epoch is January 1 1970 00:00:00 GMT. So echoing the time() function will give us “1230978041″ at the time I’m writing this, meaning that 1,230,978,041 seconds have passed since then. So why is this useful to us?

Mathematically it gives us a much easier way of keeping track of time. Sure, 2008, Jan 15th might seem all nice and organized to you, but to calculate the days passed from the 15th of Januaray to the 17th we’s have to strip the numbers of “th”, and in more complicated cases perform a bunch of other string changes. Using time you can essentially assign a number value to any given second, making it much easier to use, especially as a counter.

The place I use it most is to log out users of a website automatically after an inactivity period. When a user signs in I create all the session variables for him/her, and I also create one which holds the time his session should expire. The beauty of this whole system is that I do not need to know the actual time, I can just assign a value to the session variable like this:

$_SESSION['user']['time'] = time() +3600

This means that the user can stay logged in for 3,600 seconds (in other words an hour) from this moment in time. This is a very convenient way of defining expiration time, since you can think in terms of how long you want it to be, as opposed to trying to calculate a specific date and time.

When a user refreshes a page, or moves on to a new one, a script will check the value of the session variable. If the current time is smaller than the session variable, the user can stay logged in and I also usually prolong the session by another 3,600 seconds. This gives it the true “inactivity” aspect, since the user is allowed to stay logged in for more than an hour, as long as he/she is using the system. You could however choose not to prolong the session, in this case the user would have to log in again after one hour no matter what. In some high security systems this might be the way to go, or if you want the user to spend exactly one hour on a specific puzzle, there are many uses for everything. Needless to say that if the current time is more than the session variable the user is signed out.

Another common use for time() is to serve as the basis for generating a random ID, or character set, in other words, it can serve as a seed. A quite efficient way of creating a very random string would be to use time(), divide it by a random number generated between 0 and 9,999, add some random characters to it, and encode the whole thing using the SHA1 algorithm for example. Code-wise this is not as difficult or as long as it may sound, and it is pretty random and strong, although I am no security specialist.

if you have any cool uses for the time() function let us know in the comments!

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