 As described in the previous section, Prior introduced hybrid tense logic to deal with a particular issue in the philosophy of time, but in Prior (1968), Chapter XIV (also Chapter XIV in the new edition Prior (2003)), he also showed that hybrid tense logic can replace a two-dimensional temporal logic introduced by Hans Kamp in unpublished material circulated in 1967, and later included in Kamp (1971). The dimension is simply the number of instants a formula is evaluated relative to, so adding hybrid-logical machinery enables two dimensions to be replaced by one. This work has recently been followed up in a number of papers by Blackburn and Jørgensen, see Blackburn and Jørgensen (2016a) for an overview. We now give a brief sketch of this line of work, adapted to the terminology of the present entry. The version of hybrid logic in question has a designated nominal \(\mathtt{now}\) and each model comes with a designated time \( t_0 \) such that i) any stand-alone formula is evaluated relative to \( t_0 \) and ii) the nominal \(\mathtt{now}\) refers to \( t_0 \). More formally, we adopt the convention that \((M,t_0), g \vDash \mathtt{\phi}\) means \(M, g, t_0 \vDash \mathtt{\phi}\) and we only consider assignments \(g\) where \(g(\mathtt{now})=t_0\). Note that the nominal \(\mathtt{now}\), considered as a stand-alone formula, is valid in this semantics, but this is not the case for any other nominal. This new notion of validity is by Blackburn and Jørgensen called contextual validity. The paper Blackburn and Jørgensen (2013) gives an axiom system which is complete wrt. this notion of contextual validity. The paper Blackburn and Jørgensen (2012) gives a complete tableau system, but the semantics of this paper is in line with Kamp’s original two-dimensional semantics. Both papers also consider further indexicals like \(\mathtt{yesterday}\), \(\mathtt{today}\) and \(\mathtt{tomorrow}\). 