Table 1.

Principal categories and sub categories of temporal expressions

Category	Types	Definition	Features
Primitive of time	Point	Ideal for specifying accurate positions in time	Scheduling, planning, temporal constraints and temporal relations are barely supported [7] [8]
	Interval	Ideal for representing coarse and incomplete temporal knowledge [9]	Used by Allen’s temporal logic [10].
Linear or branching	Linear time	Time flows from past to future in a timeline order	Used by time-lining events
	Branching time	Time is linear from the past till present, after it divides into several futures	Used for hypothesizing. Suitable for diagnosis and prognosis
	Circular time	Time turns around a circle	Used to describe recurrent events, such as “administration of regular insulin every morning”
Reference of time	absolute/Anchored date and time	Accurate position in the time/day clock	Has limited temporal reasoning tasks
	Relative / unanchored date and time	More expressive, comprises more information such as temporal relation to other expressions	Entails prevalent time-based knowledge [11] and requires linguistic analysis tools
Duration of time	Quantitative duration	Fixed quantity in time	Not flexible in reasoning
	Qualitative duration	Ideal for the specification of temporal constraints	Flexible for qualitative reasoning over events and temporal expressions