SetTemporalComponent[p, val] sets the value of the temporal component of a 4-vector p, TemporalPair[ExplicitLorentzIndex[0],TemporalMomentum[p]] to val.
Overview, TC, TemporalPair, TemporalMomentum.
FCClearScalarProducts[]
ClearAll[t]
SetTemporalComponent[p, t]
TC[p]t
TC[p + q] // ExpandScalarProductq^0+t
SP[p, q] // LorentzToCartesianq^0 t-\overline{p}\cdot \overline{q}