It sounds like the date field you are getting sometimes does not match the pattern you are expecting.

For example, if the date was 2/8/2006 1:14:42 PM, then $amonth would be undef and --$amonth would render -1; hence the error message.

Also, your supplied code snippet is wrong, you are naming the day number as $ayear and the year as $aday.

You need to verify that the date is in the format you are expecting, and perhaps allow for some alternate formats.

Maybe something like (untested):

my $app_time;
if( 
    my( $aday, $amonth, $ayear, $ahour, $amin, $asec ) = 
        $appdate =~ m[
             (\d{1,2}) [-/] (\d{1,2}) [-/] (\d{4}) \s+
             (\d{1,2})  :   (\d{1,2})  :   (\d{1,2})
        ]x
) {
    $app_time = timelocal( $asec, $amin, $ahour, $aday, --$amonth, $ay
+ear );
}
else {
    die "Invalid date (format) '$app_date'";
}
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Or you could look into using one of the many date modules from cpan.