more fCalendar

It's me again, with Japanese calendar minutiae I'm sure you've all  
been dying to brush up on.

the fCalendar functions don't include the Japanese Vernal Equinox  
holiday.  this is perhaps because there is no easy way to calculate  
it.  at any rate, here's a function i wrote to fill the gap.

=====

JPVernalEquinox <- function(year) {

     ##  Origin and End Date data from http://aa.usno.navy.mil/data/ 
docs/EarthSeasons.html
     ##  The function Vernal.Equinox delivers correct values at the  
endpoints of the above data.
     ##  There may be minor variances (+/- a few minutes) in the  
intermediate values,
     ##  because the function linearly approximates a phenomenon that  
is apparently
     ##  nonlinear in recorded time.

     Equinox.Origin <- timeCalendar(1992, 3, 20, 8, 48, 0,  
FinCenter="GMT")
     Data.EndDate <- timeCalendar(2020,3,20,3,49,0,FinCenter="GMT")
     Total.Seconds <- as.numeric(Data.EndDate-Equinox.Origin)*24*60*60
     Mean.Annual.Seconds <- Total.Seconds/(atoms(Data.EndDate)$Y-atoms 
(Equinox.Origin)$Y)
     Vernal.Equinox <- function(year) Equinox.Origin+unclass((year- 
atoms(Equinox.Origin)$Y)*Mean.Annual.Seconds)
     JPVernal.Equinox <- function(year) timeDate(as.character 
(Vernal.Equinox(year)), FinCenter="Tokyo")

     ## Nota bene:  JP Vernal Equinox is celebrated when the equinox  
occurs in the Japanese time zone
     ## (see, e.g., 2006, where GMT Vernal Equinox is on 20 March,  
but Japanese Equinox holiday is 21 March)

     as.Date(as.character(JPVernal.Equinox(year)))}

=====

in contrast to the other holiday functions in fCalendar, the above  
function returns an object of class "Date" rather than "sdate"  
because use of sdates appears to be deprecated following the  
introduction of the Date class, and also because the sdate function  
appears to have a bug.
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