Note: This topic was initially included in the AIXM Temporality Concept, version 1.0.

This page explains how to interpret situations that can occur between TimeSlice types, their validity and properties with schedules. The key aspect is that the value of a property may be undefined if the schedules associated with a property leaves “holes”.

In the current AIS practice it is quite common to specify only the "main" value, such as “operational”, “active”, etc. For example, it is indicated that the “navaid operates every day from 06:00 – 22:00”, but it is not explicitly indicated what is the status of the navaid between 22:00 – 06:00. Operational people will assume that the navaid is not operating between 22:00 – 06:00. When working with digital data, machines cannot make assumptions about the value of a property in the holes left by the Timesheet.

For digital data processing, it is safer if the “non-operating” times are also stated explicitly, so that schedules associated with the values of the property do not leave any unspecified periods. Therefore, it is recommended that 'BASELINE' TimeSlices contain only fully defined properties with schedule, which indicate explicitly the property values at every moment within the validity time of the TimeSlice. If one or more Timesheet are associated with a property with schedule, then the value of the property shall be considered as undefined at any moment not covered by a Timesheet.

Timesheets that leave gaps can also occur for 'TEMPDELTA' TimeSlices. By definition, any property contained in a Tempdelta overrides the value of the equivalent Baseline property, for the duration of validity of the Tempdelta. Therefore, as explained in the Temporality Concept rules (“Delta” for complex properties), the times encoded in Timesheet contained in 'TEMPDELTA' TimeSlices also replace in full the times encoded in the equivalent 'BASELINE' Timesheets. The following example shows a hypothetical situation where the Baseline includes a property that alternates its value according to a timesheet. The following situations could occur:

Cases (the small vertical rectangles represent time intervals)	Interpretation
	TEMPDELTA with constant value (no Timesheet) overrides the BASELINE schedule
	TEMPDELTA schedule (with Timesheet) overrides the BASELINE schedule.
	TEMPDELTA schedule (with Timesheet) leaves gaps - the property is undefined in the gaps of the TEMPDELTA Timesheet This should be avoided! The temptation could be to consider that the BASELINE situation applies in the gaps left by the Timesheet(s) associated with the TEMPDELTA. This interpretation is in conflict with the general principle (as stated in the AIXM Temporality Concept, section with title “Delta” for complex properties) - TEMPDELTA values replace the BASELINE values in full. Therefore, if a TEMPDELTA schedule leaves gaps (periods for which the value is not explicitly provided), then it shall be considered that the property has an unspecified value at those time periods.

From the examples above, it is therefore recommended that TEMPDELTA schedules do not leave unspecified periods (gaps) within the time of applicability of the TEMPDELTA. In addition, data providers shall pay attention to the situation where a BASELINE is updated during the validity of the TEMPDELTA. The TEMPDELTA might need to be corrected (re-issued) if the BASELINE property affected by the schedule has changed.

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