NAME:
	arg_time
 PURPOSE:
	Converts a time argument to a time structure or Carrington variable
 CATEGORY:
	Tell time
 CALLING SEQUENCE:
	FUNCTION arg_time, ut, carrington=carrington
 INPUTS:
	ut	    array; type: time structure, float or integer
			times to be converted (UT)

			type time structure is interpreted as times in UT
			type float is interpreted as a Carrington variable
			type integer is also interpreted as a Carrington variable,
			    UNLESS all values in the input array are less/equal 12. In
			    that case the times are interpreted as a difference with the
			    system time in hours.
			if ut is undefined then the system time is returned.
 OPTIONAL INPUT PARAMETERS:
	/carrington	by default the input is converted to a time structure.
		    If /carrington is set than a Carrington variable is returned.
 OUTPUTS:
	ut	    array; type: time structure or Carrington variable,
			depending on the setting of keyword /carrington
 INCLUDE:
	@compile_opt.pro	; On error, return to caller
 CALLS: ***
	Carrington, InitVar, IsTime, IsType, TimeSystem
 PROCEDURE:
	Provides a uniform way to convert times specified as input arguments to functions
	and procedures to either time structures or Carrington variables.
 MODIFICATION HISTORY:
	MAR-2001, Paul Hick (UCSD/CASS; pphick@ucsd.edu)