The uptime value is how long the system has been running.

This value is an important parameter to observe the evolution of your system.

In Raspbian (but the following instructions work fine also on other linux flavours) you can get the uptime in different ways, my favourite ones are the following:


The simplest way

Into a bash shell issue the following command:
$ uptime
19:47  up  1:34, 2 users, load averages: 0,38 0,29 0,31

The uptime utility displays the current time, the length of time the system has been up, the number of users, and the load average of the system over the last 1, 5, and 15 minutes.


A more complete tool gives to you the uptime value
Into a bash shell issue the following command:

~ $ w
19:49  up  1:35, 2 users, load averages: 0,40 0,30 0,31
USER     TTY      FROM             LOGIN@  IDLE WHAT
gg1      pts/0    –                18:15    1:33 –
gg1      pts/2    –                19:43       – w

The who utility displays a list of all users currently logged on, showing for each user the login name, tty name, the date and time of login, and hostname if not local.
The first line of the output returns the same info of the uptime command

The programming way
if you need the uptime value within your program you have to ask for this value to the proc filesystem. 
In the /proc/ directory there is a file named (strange!!!) uptime…
the uptime file contains two float values, the first is the uptime value while the second  is how much time the system has spent idle. Both the values are expressed in seconds.
So from a c program you can do an fscanf("%f %f) on the /proc/uptime file.