These drawbacks of ISO-TimeML, especially the lack of compositionality, make the development of machine learning models difficult.
most prior work has taken a rule-based approach, looking up each token of a time expression in a normalization lexicon and then mapping this sequence of lexical entries to the normalized form (Strötgen and Gertz, 2013;Bethard, 2013;Lee et al., 2014;Strötgen and Gertz, 2015).