ut_calendar and negative times

Hi,

Leaving my 64-bit issues aside for the moment and returning to my trusty G5 Mac, still running NCL 5.1.1 though.

I've found what may be a bug in ut_calendar (or may be my idiocy of course).

I found that when it encountered negative values in the time field, it gave false results.

Here is an example.

We have a monthly 'time' variable, the first element and attributes are as follows:

ncl 6> print (time(0))

Variable: time (subsection)
Type: float
Total Size: 4 bytes
            1 values
Number of Dimensions: 1
Dimensions and sizes: [1]
Coordinates:
Number Of Attributes: 5
  time : -39615
  axis : T
  calendar : 360_day
  units : days since 1970-01-01 00:00:0
  long_name : Time
(0) -39615

So we see it's 360 day years (that's not unusual for climate model work). The reference day is Jan 1st 1970, and the first time point is -39615.

Remember it's monthly data, that's why the number ends in '15' (months are usually represented by a day roughly in the middle of the month).

If you examine -39615, it turns out to be 110 years and 15 days (in 360-day years). So the first datum represents Jan 15th 1860, or 'January 1860', which is as expected.

But what happens when we give ut_calendar this?

ncl 9> date = ut_calendar(time(0),-1)
ncl 10> print(date)

Variable: date
Type: integer
Total Size: 4 bytes
            1 values
Number of Dimensions: 1
Dimensions and sizes: [1]
Coordinates:
Number Of Attributes: 1
  calendar : 360_day
(0) 172201

The date returned is January 1722.

So, is this a known problem? Or have UKMO crash-tested ut_calendar with their archiving of the HadGEM model output?

Or am I being an idiot again? :)

Cheers

Harry

Ian "Harry" Harris
Climatic Research Unit
School of Environmental Sciences
University of East Anglia
Norwich NR4 7TJ
United Kingdom

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Received on Wed Jan 13 11:10:32 2010