;+
; NAME:
;	TimeReadMe
; PURPOSE:
;	Help file for time variables in SMEI IDL software
; CATEGORY:
;	gen/idl/toolbox/time
; SEE ALSO:
;	defsysv_smei, TimePieces
; PROCEDURE:
;	Three system variables are defined at startup (in defsysv_smei.pro)
;	to control the handling of time variables:
;
;	IDL> help, /structure, !timeunits
;	** Structure TIMEUNITS, 2 tags, length=8, data length=8:
;		IN_DAY          LONG          86400000
;		TINY            LONG          86400000
;	IDL> help, /structure, !thetime
;	** Structure THETIME, 2 tags, length=8, data length=8:
;		DAYS            LONG                 0
;		UNITS           LONG                 0
;	IDL> help, /structure, !timezero
;	** Structure THETIME, 2 tags, length=8, data length=8:
;		DAYS            LONG                 0
;		UNITS           LONG                 0
;
;	!TimeUnits has two integer fields:
;
;	!TimeUnits.in_days defines the unit used to keep track of the time of day
;		in !TheTime.units. It is the number of these time units in one whole day.
;		Currently this unit is milli-second, i.e. !TimeUnits.in_day = 86,400,000
;		Other allowed values are 24 (time-of-day units are hours), 1440 (units are
;		minutes) or 86400 (units are seconds).
;
;	!TimeUnits.tiny defines the smallest allowed time units allowed for the
;		time of day. Currently this also is milli-second, i.e.
;		!TimeUnits.tiny=!TimeUnits.in_day.
;
;	!TimeUnits.tiny is used primarily in rounding operations
;	(see the /roundt keyword on TimeGet).
;
;	All time variables are defined as arrays of !TheTime structures.
;
;	!TheTime has two long integer fields that together define the time elapsed since
;	some reference time (time origin).
;
;	!TheTime.days keeps track of integer days.
;	!TheTime.units keeps track of the time of day in units defined by !TimeUnits.in_day.
;
;	i.e. the time (in days) elapsed since the reference time is
;		!TheTime.days+!TheTime.units/!TimeUnits.in_day.
;
;	By default the reference time is set in !TimeZero.
;	!TimeZero itself is a !TheTime structure, and defines the time elapsed since
;	the 'absolute time origin' 2000, Jan 1, 0 UT. The default for !TimeZero is {0,0}
;	i.e. the current origin is set by default to the absolute origin.
;
;	TimeOrigin:
;
;	The function TimeOrigin is used to manipulate !TimeZero, and the setting
;	of !TimeUnits.in_day (!TimeUnits.tiny is never changed).
;
;	TimeShift:
;
;	A change of origin usually means that time variables need to be modified to
;	reflect the new origin. This is done with TimeShift.
;
;	TimeSet:
;
;	Most common real-life times can be converted to a !TheTime structure using
;	TimeSet. Examples are yr,mon,day triplets, yr,doy pair, (Modified) Julian Days,
;	Julian epochs, various string formats, etc.
;
;	TimeGet:
;
;	This function is used to extract real-life data from a !TheTime structure.
;	Examples are year, day of year, day of week, day of month, Julian day, etc.
;	It also provides additional services, such as rounding of times.
;
;	Usage:
;
;	In principle, every routine that takes a time structure as input,
;	should assume that the time specifies time elapsed since the origin
;	!TimeZero.
; MODIFICATION HISTORY:
;	FEB-2004, Paul Hick (UCSD/CASS; pphick@ucsd.edu)
;-

; No code in this file