Explicit expressions can be directly normalized with standard temporal knowledge (e.g., 2016-01 for January 2016) and implicit expressions with non-standard temporal knowledge such as information about holidays (e.g., 2016-03-17 for Saint Patrick's Day 2016).
the normalization of relative and underspecified expressions requires a reference time for the normalization, and a relation to the reference time for underspecified expressions, additionally.