DTV12 — Gregorian years before 1600 have no starting day

Specification: DTV v1.1

Title: Gregorian years before 1600 have no starting day

Summary:

In clause 11.7, in the entry for ‘Gregorian year has starting day’, the Definition is a formula for calculating the index of the ‘starting day’, as distinct from a definition of the verb concept (which is actually carried in the definition of the role ‘starting day’). The Definition is followed by the Necessity:

Necessity: The index of Gregorian year is greater than 1600.

We must conclude that either there are no Gregorian years before 1600, or the ones before 1600 have no starting day. The real intent is probably that the formula given in the Definition is not correct for Gregorian years before 1600.

The correct definition is: The starting day is the Gregorian day that is the first calendar day of the Gregorian year.

Necessity: Each Gregorian year has exactly one calendar day.

Necessity: The starting day of Gregorian year 1875 is Gregorian day 684606.

This relates the concept to the index origin for the Gregorian days scale. The rest are determined by the rules for the length of Gregorian years.

How to compute the index of the starting day from 1601 on should be a Note. One would expect the rule to be how to calculate from the index origin, and there may be other such algorithms. The concept is not the computation algorithm.

The RTF agrees that the definition should be corrected, and the first Necessity should be added.
The Gregorian calendar algorithm is, however, a Necessity not a Note, and the starting day of 1601 is important to it. The second proposed Necessity is irrelevant – the index origin of the Gregorian days calendar is not the starting day of any Gregorian year, and is not useful to the calendar algorithm.
Note also that there are no Gregorian years before 1582. That Necessity is corrected