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We present an approach to time normalization (e.g. the day before yesterday ⇒2013-04-12) based on a synchronous context free grammar. Synchronous rules map the source language to formally defined operators for manipulating times (FindEnclosed, StartAtEndOf, etc.). Time expressions are then parsed using an extended CYK+ algorithm, and converted to a normalized form by applying the operators recursively. For evaluation, a small set of synchronous rules for English time expressions were developed. Our model outperforms HeidelTime, the best time normalization system in TempEval 2013, on four different time normalization corpora.