The BNF grammar in Figure 4 describes the core set of the temporal expressions handled by our temporal reference resolution system. To simplify the grammar, some variations are not specified. Combinations of the expressions are also allowed, provided that this doesn't lead to contradictory temporal interpretations. This is based on English transcription of the Spanish phrases that are covered by the CMU semantic parser. Note that conjunction is allowed, although not specified here. Also, brackets are used for optional components, and parentheses are used for grouping items.