Temporal expressions


A temporal expression in a text is a sequence of tokens that denote time, that is express a point in time, a duration or a frequency.
Examples:
He was born on 6 May, 1980.

The show lasted 7 minutes.

The pump circulates the water every 2 hours.

Initially, temporal expressions were considered a type of named entities and their identification was part of the named entity recognition task. Since the Automatic Content Extraction program in 2004 there has been a separate task identified and called Temporal Expression Recognition and Normalisation. Timex evaluation is now evaluated in two major temporal annotation challenges: TempEval and i2b2, both of which prefer the TimeML-level TIMEX3 standard.

Approaches

Similarly to NER systems, temporal expression taggers have been created either using linguistic grammar-based techniques or statistical models. Hand-crafted grammar-based systems typically obtained better results, but at the cost of months of work by experienced linguists. There are many such systems available now, so creating a temporal expression recognizer from scratch is generally an undesirable duplication of effort. Instead, current approaches focus on novel subclasses of timex.
Statistical systems typically require a large amount of manually annotated training data and are usually applied to the recognition task only.