# Calculating entropy change

Entropy is a state variable, i.e. the data about the initial and final states would suffice to define change in entropy . However, while applying the formula dS = integral of dQ/T, I have distinctly been told that dQ is to be calculated by assuming a hypothetical reversible process, irrespective of whether we are given a reversible or an irreversible process in the problem. Although heat depends on the thermodynamic path, S doesn't. So why should it matter whether the process is irreversible, whether Q is calculated for an irreversible process, as S doesn't depend on all this? (I know that the math won't work out and we'll end up with different values, but my question is a conceptual one.)