  Estimation of planetary orbital and physical parameters from light-curve data
relies heavily on the accurate interpretation of Transit Timing Variations
(TTV) measurements. In this letter, we review the process of TTV measurement
and compare two fitting paradigms - one that relies on making
transit-by-transit timing estimates and then fitting a TTV model to the
observed timings and one that relies on fitting a global flux model to the
entire light-curve data simultaneously. The latter method is achieved either by
solving for the underlying planetary motion (often referred to as
"photodynamics"), or by using an approximate or empirical shape of the TTV
signal. We show that across a large range of the transit SNR regime, the
probability distribution function (PDF) of the mid-transit time significantly
deviates from a Gaussian, even if the flux errors do distribute normally.
Treating the timing uncertainties as if they are distributed normally leads, in
such a case, to a wrong interpretation of the TTV measurements. We illustrate
these points using numerical experiments and conclude that a fitting process
that relies on a global flux fitting rather than the derived TTVs, should be
preferred.
