DTV — DTV Issue: A calendar day is not a time period

Specification: Date Time Vocabulary
Version: Beta-1
Title: A calendar day is not a time period
Source: Ed Barkmeyer, NIST, edbark@nist.gov

Summary:

DTV section 9.5.3 defines 'calendar day', 'calendar month' and 'calendar year' to be time points, as shown in figure 9.10.
The entry for 'day period', however, contains a Note that reads: "Calendar day is a period that starts and ends as defined by a calendar. Day period starts and ends at any time within a calendar day."
And there are similar notes under 'month period' and 'year period'.

A calendar day is not a '(time) period'; it is a 'time point'. But a 'day period' time interval can indeed start and end at any time point "within", i.e., on some time scale that subdivides, a calendar day.

The Note could be modified to read: "A calendar day corresponds to time intervals that start and end as defined by a calendar." Or, the concept "calendar day period" could be introduced to refer to time intervals that instantiate calendar days. The juxtposition of the two otherwise unrelated sets of concepts in Figure 9.10 suggests that the latter may be what was intended. And in any case, the repair must be applied to year period and month period as well.

It appears that a 'day period' is not just a time interval whose duration is one day, because of leap seconds. A note to that effect would be valuable. (It is clear that months and years are of variable duration.)

The Notes are incorrect and are reworded, essentially as suggested. The FTF agrees that Figure 10.2 (formerly 9.10) suggests a relationship between two sets of concepts that was not intended, and that two separate diagrams are wanted. The proposed Note about leap seconds is broadened to include any changes in time offset that affect the definition of local time of day on consecutive days.