Goal: Learn about time axis in CDAT.

The time axis is somewhat special, because it holds the time object.

Let's start by opening a file we will query

import cdms,sys,cdtime
f=cdms.open(sys.prefix+'/sample_data/clt.nc')

and let's get some variable, for example variable 'v'

v = f('v')       # transient variable 'v' 

then get the time axis

t=v.getTime()    # return the time axis
print t              

  id: time2
    Designated a time axis.
    units: months since 1978-12
    Length: 1
    First: 1.0
    Last: 1.0
    Other axis attributes:
      calendar: proleptic_gregorian
      axis: T
    Python id: -0x48f56974

To get the values just type:

print t[:]    # print values of the time axis

     [ 1.,]

When we printed the time axis variable 't' (using 'print t[:]') we got a value '1.'. If you also recall the info on time axis (that we got from 'print t' command), you will realize that the value '1.' means 'first month from the fixed start time', which in this case is '1.0 months since 1978-12' i.e. '1979-01-01' .  

Of course, you can get this info directly from CDAT :

time_comp =  t.asComponentTime()[:]    #print time in a more user friendly way
print time_comp

   [1979-1-1 0:0:0.0]

Here is how you can go back and forth from so called 'component time' and a 'relative time'. 

A component time is an absolute time with representation (year, month, day, hour, minute, second). A relative time is represented as (value, units since basetime).

The component time is in this case '[1979-1-1 0:0:0.0]' and the relative time is '1.00 months since 1978-12'

To create the time in as a relative time, we need the units we want to use, and a time value.

t_units = t.units   # get time 't' units
print t_units

months since 1978-12

Now let's take the first value:

t0 = tim[0]
print t0 

   1.0

And for the last value, just do:

t_last = tim[-1]

Now we can create the relative time:

time_rel = cdtime.reltime(t0, t_units) # t0 only rank 0 array (scalar only!!)
print time_rel

    1.00 months since 1978-12

To convert it to a component time, use:

time_comp = time_rel.tocomp()   # convert relative time to component time
print  time_comp

1979-1-1 0:0:0.0

You can learn more about other ways of querying the files, variables, dimensions and axis by scanning through the Quick Reference Guide to CDMS or by following the CDMS Basics tutorial.

The CDAT templates plots images and the python scripts can be found here.