Re: [ferret_users] 360_day calendar annual mean

[Ansley Manke <ansley.b.manke@xxxxxxxx>]

Hi -
Here is a question that was sent just to me, but it's of general 
interest, so I'll post my answer below.

On 3/16/2012 11:35 AM, Kelin Zhuang wrote:
> Dear Ansley,
> When I try to make an annual of a variable from 1985 to 2004, I use 
> the following algorithm:
> define axis/calendar=360_day/edges/t0=1-jan-1900/units=days  
> timeline=days1900(x[i=1985:2005:1],1,1)
> let y=x[gt=timeline@ave]
> But I got the first data as 29-sep-1986 and
>  SEP-1986 /  1:   1.00
>  OCT-1987 /  2:   2.00
>  OCT-1988 /  3:   3.00
>  OCT-1989 /  4:   4.00
>  OCT-1990 /  5:   5.00
>  OCT-1991 /  6:   6.00
>  OCT-1992 /  7:   7.00
>  NOV-1993 /  8:   8.00
>  NOV-1994 /  9:   9.00
>  NOV-1995 / 10:  10.00
>  NOV-1996 / 11:  11.00
>  NOV-1997 / 12:  12.00
>  DEC-1998 / 13:  13.00
>  DEC-1999 / 14:  14.00
>  DEC-2000 / 15:  15.00
>  DEC-2001 / 16:  16.00
>  DEC-2002 / 17:  17.00
>  DEC-2003 / 18:  18.00
>  JAN-2005 / 19:  19.00
>  JAN-2006 / 20:  20.00
> That's not what I want. When using gregorian or other calendars, I can 
> get Jul16 of every year. Thanks for your help in advance,
>
> Best regards,
> Kelin

The function DAYS1900 is based on Ferret's default Proleptic Gregorian 
calendar, so it isn't what you want to use to define the timesteps for a 
360-day calendar axis. Better to construct the times directly. This is 
easy because every year is the same length.  You can put the time origin 
anywhere, but it could go something like this:

yes? define axis/calendar=360_day/edges/t0=1-jan-1984/units=days  
timeline=360*t[t=1:29]
Replacing definition of axis TIMELINE
yes? list t[gt=timeline]
             VARIABLE : T
                        axis TIMELINE
             SUBSET   : 28 points (TIME)
             CALENDAR : 360_DAY
 JUL-1985 /  1:    540.
 JUL-1986 /  2:    900.
 JUL-1987 /  3:   1260.
 JUL-1988 /  4:   1620.
 JUL-1989 /  5:   1980.
 JUL-1990 /  6:   2340.
 JUL-1991 /  7:   2700.
 JUL-1992 /  8:   3060.
 JUL-1993 /  9:   3420.
 JUL-1994 / 10:   3780.
 JUL-1995 / 11:   4140.
 JUL-1996 / 12:   4500.
 JUL-1997 / 13:   4860.
 JUL-1998 / 14:   5220.

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