Time can specify a validity period of a fact or opinion, or it can specify a begin or end or duration of an occurrence.
Depending on these situations time is modeled in different ways.
Every fact (or opinion) has a validity period, also called its life time. A fact may have been the case 'always' and may last 'for ever', but it may also have a limited validity period and may change continuously, such as being a measurement of a rapid changing aspect (such as a velocity or temperature), which value is already replaced by another value within a fraction of a second. Thus not only the existence of physical objects, but also the validity period of aspect values need to be modeled and recorded.
The existence of something that appears in a data set starts its life time at the 'date-time of the start of validity' of its classification relation or specialization relation, or one of their subtypes.
The end of its existence period or validity period may be specified explicitly by specifying its date-time of latest change, which indicates the termination of its life when the change includes that the status changes into deleted or replaced or history. The time can be recorded as precise as required, because it is specified as a decimal number (according to the standard '1900 date system' notation).
Every fact in a Gellish data set has in principle an accessory fact that specifies its date-time of start of validity and an accessory fact that enables to record the date-time when its validity terminates. The latter is recorded as the date-time of latest change, in combination with a status deleted, replaced or history for facts that are not valid any more, or have been valid in the past.
In addition to that it can be specified since which date-time a fact became available in the database and at which date-time a particular copy of the expression was made.
In a Gellish Expression Table these four date-time values can be recorded in four separate columns.
In addition to that, it is possible to model the begin or end or duration of occurrences, the period (or date-time) within which something occurs. For example, a possible expression would be:
Note that a date implies a period of 24 hours. Things that are supposed to happen at a particular moment in time are assumed to occur within a very short period in time, typically denoted by only one value. For example, When something is said to occur at 12.15 h, it means that it occurs within the duration of one second, which starts at 12.15 h. Short events typically have an unspecified duration, although their duration and possible accuracy of measurement can be specified explicitly.
Note: These ways of modeling time mean that it is not needed to create separate 'temporal part' objects for the duration of particular situations (states), such as is required for models according to ISO 15926-2. Nevertheless, Gellish also allows for the creation of temporal-part objects, called a 'physical object in state'. Logic reasoning enables to convert from one way of modeling to the other, if required.
Further guidelines on the modeling of time and measurements are given in the full article and in the Gellish Modeling Methodology Part 9, Measurements and Observations (which can be purchased via the webshop or are available for free for licensees).