Q44305: TIME$ Problem in IF Statement at Midnight (24:00:00)

Article: Q44305
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom | mspl13_basic
Last Modified: 20-SEP-1990

The following program demonstrates a possible programming mistake with
the TIME$ function in the Microsoft QuickBASIC environment. If you
test the TIME$ function twice in one IF statement, the returned time
may change between invocations and give you a conditional result that
you did not expect. In the example below, the symptoms occur only
intermittently during the tenth of a second at midnight (24:00:00). To
correct this programming mistake, you should assign the value returned
by TIME$ to a temporary string variable, which can then be reliably
tested multiple times in the IF statement.

This information applies to Microsoft QuickBASIC versions 4.00, 4.00b,
and 4.50 for MS-DOS; to Microsoft BASIC Compiler versions 6.00 and
6.00b for MS-DOS; and to Microsoft BASIC Professional Development
System (PDS) versions 7.00 and 7.10 for MS-DOS. (Programs compiled
with BC.EXE may not be as sensitive to this timing issue as the QB.EXE
or QBX.EXE environment.)

When run inside the QuickBASIC environment (QB.EXE), the two TIME$
functions in the following program may return different values
(depending upon timing) and sometimes make the IF statement STOP the
program. You might think that the IF statement should never be
executed, and the programmed loop should continue until you press
CTRL+BREAK. But after the program runs long enough, it will eventually
show the behavior in QB.EXE or QBX.EXE. This behavior may not occur in
the executable version (.EXE) of the program.

To correct this programming error, assign a string variable to the
TIME$ function and execute the IF statement using this variable
instead of TIME$, as noted in the commented portions of the program
below.

The following code demonstrates this behavior. At 24:00:00, this
program will (sometimes) unexpectedly execute the IF statement. The
faster the machine, the more likely the behavior will show up. The
behavior occurs only during the tenth of a second at midnight, since
at that time, the TIME$ function may return 24:00:00 and then 00:00:00
at the next invocation. You may have to run the program a number of
times to demonstrate the behavior, since it does not occur every time.

CLS
MISSED:
       TIME$ = "23:59:00"
START:
       '*** The IF statement below sometimes executes, even though
       '*** you may have thought it was not logically possible:
       IF TIME$ >= "10:00:00" AND TIME$ < "16:59:58" THEN STOP
       'To correct this behavior, replace the line above with the
       'following two lines:
       '  TimeVar$ = TIME$
       '  IF TimeVar$ >= "10:00:00" AND TimeVar$ < "16:59:58" THEN STOP
       LOCATE 3, 1
       PRINT TIME$
       '*** If program didn't stop, reset time and try again:
       IF MID$(TIME$, 5, 1) = "1" THEN GOTO MISSED
       GOTO START

If you implement the correction shown above, the program will not stop
in QB.EXE or QBX.EXE until you press CTRL+BREAK.

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