'===========================================================================
' Subject: 2 DIGIT YEARS                      Date: 03-10-00 (20:19)       
'  Author: David O. Williams                  Code: QB, QBasic, PDS        
'  Origin: david.williams@ablelink.org      Packet: DATETIME.ABC
'===========================================================================
' 2dig_yr  David O. Williams  2000

' E-mail: david.williams@ablelink.org

' Public Domain.  No cost.  No warranty.  (But I think it is right!)

' Program is explained in PRINT statments below.


DEFINT A-Z
DECLARE FUNCTION LongYear (X)
CLS

PRINT "This program demonstrates present-centred two-digit year dating."
PRINT
PRINT "For many purposes, dates more than twenty or thirty years into"
PRINT "either the past or the future are virtually never encountered."
PRINT "It is therefore possible to use just two digits to represent"
PRINT "the year, provided they are always interpreted in the context"
PRINT "of the present date.  For example, in the year 2000, the digits"
PRINT "'75' would be interpreted as meaning the year 1975.  But in"
PRINT "2050, the same two digits would mean the year 2075."
PRINT
PRINT "To perform this interpretation, this program includes a function"
PRINT "called LongYear.  If the argument that is passed into this"
PRINT "function is an integer between zero and 99, inclusive, so it"
PRINT "can be a two-digit year, the LongYear function takes a value"
PRINT "that is between (P-49) and (P+50), inclusive, where P is the"
PRINT "*present* year, as shown by the computer's internal calendar,"
PRINT "and the right two digits of the function output are the same as the"
PRINT "two digits of the input argument."
PRINT
PRINT "To demonstrate this function, the following loop allows you to"
PRINT "input two-digit years, and will output them in four-digit format."
PRINT "Enter any negative number to quit."
PRINT

DO
 INPUT "Year"; Y
 IF Y < 0 THEN EXIT DO
 PRINT LongYear(Y)
LOOP

CLS
PRINT "In order to use this dating system in any program, just use"
PRINT "two digits for the year-number in any date.  However, before any"
PRINT "calculations, comparisons, etc., are done that involve dates,"
PRINT "pass the two-digit values through the LongYear function, and"
PRINT "do the calculations with the four-digit outputs of the function."
PRINT

END

FUNCTION LongYear (X)
  Y = X
  IF Y < 100 AND Y >= 0 THEN
     Y = ((VAL(RIGHT$(DATE$, 4)) + 50 - Y) \ 100) * 100 + Y
  END IF
  LongYear = Y
END FUNCTION