Re: Calendar programming

On Sun, Apr 25, 2004 at 07:17:59PM -0400, Roger Mills wrote:
> Getting into specific time of day is harder but not impossible; but math
> errors tend to creep in....
Indeed.  Time of day complicates things even for Earth calendars, since
the civil day begins at different times relative to the sun in different
calendars: midnight (that is, the midpoint between successive noons,
which is similar but not identical to the midpoint between sunset and
sunrise) in the Gregorian calendar, sunrise in the Hindu calendars, noon
in the Julian Day count, sunset in the Hebrew and Islamic calendars . . .

And time zones further complicate things.  The Julian Day represents
absolute time; as I type this sentence, it is JD 2,453,121.53381,
no matter where on Earth you might happen to be, because the Julian Day
begins at noon UTC.  In most calendars the civil day begins at
some designated point of the day in LOCAL time, so using the JD as
a go-between can complicate things.

Incidentally, I have a page at
http://thereeds.org/~mark/software/calendars.rhtml where you can convert
your birthdate into various calendars and find out when the next
anniversary is according to each of them, and how old you'll be
according to each one's idea of what constitutes a "year".  Because
of the considerations outlined above, you do need to know what time of
day you were born, at least to the nearest quarter day.

-Mark