Is it possible to determine the absolute value of entropy of a system? I came across a question (in some assignments I had received) where it was asked: 


  Can the absolute value of entropy of a system can be measured?


and to my surprise, the answer said yes. Till now, I knew that entropy is a state function, just like enthalpy and internal energy, and that only change in entropy can be measured. 

Am I missing something here?
Well, in practical I would say it is not possible. But there is a way to theoretically approximatly calculate an absolute value for entropy. 

According to the third law of thermodynamics the entropy change can be calculated as 

$$\mathrm d S = \frac{\delta q}{T}\, .$$

Per definition, a perfect crystal at $\pu{0 K}$ would have an entropy $S=0$.

So when you now heat that crystal and measure the change in entropy, since your initial $S$ was zero, the change is equal to the absolute value.

One problem is though, that the above equation only holds for constant temperatures. So the steps at which you measure the entropy changes have to happen at very (infinitesimal) small temperature changes.