Re: OT: CONCULTURE: Ayeri calendar again

On Mon, Jan 10, 2005 at 06:20:28PM +0100, Carsten Becker wrote:
> Hey all, especially Mark J. Reed -- I need your help as a
Uh-oh, I've been singled out!  The Powers know where I am!  Guess
it's time to pack up and change identities again . . .
I only have a minute to respond right now, but if you need more help
let me know and I'll revisit.

:)

> >    # Correspondence point
> >    $TIME_T_0 = 62772584;  # UNIX time_t of correspondence
> >                           # point  => in days
> Why do we need to know this?
For calendar conversion, what you need is a known point of
correspondence; some date in calendar A whose date in calendar B you
already know.  You not only need both dates, but you also need to know
the relative lengths of the units, etc.

Often, the conversion process is made simpler by use of a third calendar
system to use as an intermediary, especially if this third calendar
system is really gosh-darn simple, such as a monotonically increasing
count in a single unit.  For this reason, the Julian Day is a common
choice; the emacs calendar system uses a similar system (absolute day
count) with a different starting point (Jan 1, 1 CE Gregorian).  This
simplifies the math; plus, if you wish to convert among N calendar
systems, you only need 2N sets of conversion formulae (from each
calendar to the intermediary, and the reverse) instead of N^2.

In PHP, as in many other programming languages which were born on UNIX,
dates/times are natively manipulated in "time_t" form, which is
effectively just such a third calendar system.  Time zero is January
1st, 1970 at midnight UTC(=GMT) and you just count forward from there.
The basic unit is the second, but it's easy to handle any unit that's
always the same number of seconds (ignoring leap seconds), like
microseconds, milliseconds, minutes, hours, days, or weeks.  Just not
months or years, because their size varies.

So.  Given a point in time whose date you know in both our calendar and
the Ayeri's, it simplifies the conversion if you know the time_t value
of that point.  Which is easy to get from PHP if you know the date in
the Gregorian calendar:

<?php
    $time_t = gmmktime($hour, $minute, $second, $month, $day, $year);
?>

In the earlier code I used days instead of seconds for two reason:

1. to keep the numbers to a more reasonable size
2. the day is the basic unit of most calendars

<?php
    $time_t_days =
        gmmktime($hour, $minute, $second, $month, $day, $year)/86400.0;
?>

-Mark