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<Paper uid="W06-0902">
  <Title>Local Semantics in the Interpretation of Temporal Expressions</Title>
  <Section position="3" start_page="0" end_page="9" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> Obtaining a precise semantic representation for utterances related to time is interesting both from a theoretical point of view, as there are many complex phenomena to be addressed, and for purely practical applications such as information extraction, question answering, or the ordering of events on a timeline.</Paragraph>
    <Paragraph position="1"> In the literature, work on the interpretation of temporal expressions comes from two directions.</Paragraph>
    <Paragraph position="2"> On the one hand, work in formal semantics (see, for example, (Pratt and Francez, 2001)) aims to provide a formally well-grounded approach to the representation of the semantics of these expressions, but such approaches are difficult to scale up to the broad coverage required for practical applications; on the other hand, work that has its roots in information extraction, while it emphasizes broad coverage, often results in the use of ad hoc representations. The most developed work in this direction is focused around the TimeML markup language1 (described, for example, in (Pustejovsky et al., 2003) and in the collection edited by Mani et al. (2005)).</Paragraph>
    <Paragraph position="3"> Some work attempts to bring these two traditions together: notable in this respect is Schilder's (2004) work on temporal expressions in German newswire text, and Hobbs and Pan's (2004) work on the axiomatisation in terms of OWL-Time. Saquete et al. (2002) present an approach that views time expressions as anaphoric references.</Paragraph>
    <Paragraph position="4"> We take the view that an important step towards a truly broad coverage yet semantically well-founded approach is to recognize that there is a principled distinction to be made between the interpretation of the semantics of a temporal expression devoid of its context of use, and the fuller interpretation of that expression when the context is taken into account. The first of these, which we refer to here as the local semantics of a temporal expression, should be derivable in a compositional manner from the components of the expression; determining the value of the second, which we refer to as the global semantics of the expression, may require arbitrary inference and reason1Note that with TimeML one can annotate not only temporal expressions, but also events and relations between events and temporal expressions.</Paragraph>
    <Paragraph position="5">  ing. Such a distinction is implicit in other accounts: Schilder's (2004) use of lambda expressions allows representation of partially specified temporal entities, and the temporary variables that Negri and Marseglia (2005) construct during the interpretation of a given temporal expression capture something of the same notion.</Paragraph>
    <Paragraph position="6"> Our proposal here is to reify this level of intermediate representation based on a formalization in terms of recursive attribute-value matrices. This has two distinct advantages: it provides a convenient representation of underspecification, and it leads naturally to a compositional approach to the construction of the semantics of temporal expressions via unification. We also provide a compact encoding of this representation that is essentially an extension of the existing TIMEX2 representation for temporal expressions. This brings the advantages that (a) existing tools and machinery for evaluation can be used to determine how well a given implementation derives these local semantic values; and (b) performance in the determination of local semantics and global semantics can be tested independently. To ensure breadth of coverage, we have developed our representation on the basis of the 256 examples of temporal expressions provided in the TIMEX2 guidelines (Ferro et al., 2005). To make it possible to compare systems on their performance in producing these intermediate representations, we make available this set of examples annotated in-line with the representations described here.</Paragraph>
    <Paragraph position="7"> The rest of this paper is structured as follows. In Section 2, we describe the architecture of DANTE, a system which embodies our approach to the detection and normalisation of temporal expressions; in particular, we focus on the architecture employed in this approach, and on the particular levels of representation that it makes use of. In Section 3, we argue for an intermediate representational level that captures the semantics of temporal expressions independent of the context of their interpretation, and introduce the idea of using recursive attribute-value matrices to represent the semantics of temporal expressions. In Section 4, we provide an encoding of these attribute-value matrices in a compact string-based representation that is effectively an extension of the ISO-based date-time format representations used in the TIMEX2 standard, thus enabling easy evaluation of system performance using existing tools. In Section 5 we discuss how the approach handles constructions that contain one TIMEX embedded within another. Finally, in Section 6 we draw some conclusions and point to future work.</Paragraph>
  </Section>
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