error in mysql_to_unix

Hi, there!
I had an error in mysql_to_unix

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Message: mktime() expects parameter 1 to be integer, string given
Filename: helpers/date_helper.php
Line Number: 397

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I found that line in a system/helpers/date_helper.php
The problem in this code

Code:

        return mktime(
            substr($time, 8, 2),
            substr($time, 10, 2),
            substr($time, 12, 2),
            substr($time, 4, 2),
            substr($time, 6, 2),
            substr($time, 0, 4)
        );

After included all substr in intval the error is gone.
I use the 3.0.6 version, but 3.1.2 have the same problem.