* Example: setting up a regular time axis in a descriptor
* ( see parameters D_T0TIME,D_TIME_UNIT,S_START,S_END in the descriptor file )
*	Suppose we wish to construct a time axis from "1-JAN-1980":00:00:00"
*	to "1-FEB-1980:00:00:00" by days.
*    1) First we select a date to correspond to the value T=0.
*	In planning for future analyses which may extend
*	our data set backwards in time it is preferable to place this date
*	very far back in time - since negative time values are inadmissible to
*	FERRET.  Say, lets use "1-JAN-1920:00:00:00".
* 	Insert into the descriptor file: D_T0TIME = '1-JAN-1920:00:00:00' .
*    2) Next we need to select the units of the time axis.  Since we want our
*	time axis in days a sensible choice is to use days as the unit
*	although hours, seconds, etc. will also work.  Seconds are the
*	fundamental working units for FERRET so with 60*60*24=86400 seconds
*	per day we insert into the descriptor file: D_TIME_UNIT = 86400.
*	Since each time step of our (daily) data set will be 1 unit apart we
*	also initialize "tinc" to 1., below.
*    3) Now we need to know the time step value of the start of our desired
*	time interval.  This will be the number of days between "1-JAN-1920"
*	and "1-JAN-1980" - awkward to determine.  We can get FERRET, itself,
*	to assist with this by giving the FERRET commands
*yes? define axis/t0="1-JAN-1920"/T="1-JAN-1980":"1-FEB-1980":1/UNITS=DAYS ttest
*yes? define grid/t=ttest gtest
*yes? show grid/L=1 gtest
*	FERRET will return with 
*          L     T                 BOX_SIZE       TIME_STEP (DAYS)
*          1>  01-JAN-1980 00:00     1              21915
*	telling us that the time step value of 1-JAN-1980 is 21915.
*	Use this value in t1, below.
*	Then find the last time step with the command
*yes? show grid/L=1 gtest
*	Use this value in tn, below.
*    4) Finally we need to tell the descriptor the range of time steps located
*	in each data file of the data set.  This seems unnecessary but recall
*	that GT data sets may, in general, split the time steps several data
*	files.
*	Enter in the descriptor S_START = 21915. ,  S_END = 21946.
*	and S_DELTA = 1. (for one time unit per step).
*	If the time axis is irregular omit S_DELTA.