  Wigner's form of the local realist inequality is used to derive its temporal
version for an oscillating two-level system involving two-time joint
probabilities. Such an inequality may be regarded as a novel form of the
Leggett-Garg inequality (LGI) constituting a necessary condition for
macrorealism. The robustness of its quantum mechanical (QM) violation against
unsharpness of measurement is investigated by using a suitable model of unsharp
measurements. It is found that there exists a range of values of the sharpness
parameter (characterizing precision of the relevant measurements) for which the
usual LGI is satisfied by QM, but Wigner's form of the LGI (WLGI) is violated.
This implies that for such unsharp measurements, the QM violation of
macrorealism cannot be tested using the usual LGI, but can be tested using
WLGI. In showing this, we take into account the general form of the usual LGI
involving an arbitrary number of pairs of two-time correlation functions.
Another recently proposed necessary condition for macrorealism, called
`no-signalling in time', is also probed, showing that its QM violation persists
for arbitrarily unsharp measurements.
