DTV11 — incorrect formula for length of successive Gregorian years

The formula given for the possible lengths
for an integral number of successive
Gregorian years is incorrect. It implies
an average length of the year of 365.235 d
while the correct value is 365.2425 d.
For instance it is well known that any
successive 400 Gregorian years comprise
exactly 146 097 d, while the formula
gives 146 094 d.
For the record, with a little algebra,
the number of days between Gregorian
calendar(Y', Jan, 01) and Gregorian
calendar(Y, Jan, 01) is easily seen
to be
floor( Δ·365.25 )
+ floor(-3/4·floor(Δ/100))
+ |Δ mod 4 > (Y - 1)mod 4|

• Δ mod 100 > (Y - 1)mod 100

  + |Δ mod 400 > (Y - 1)mod 400|
  where Δ = Y - Y'.

The reference is to the formula in the entry for ‘year value specifies duration value set’. The formula includes two erroneous factors of 2, and does not account for year values greater than 400 properly. The formula and the supporting concepts will be corrected.