creating a unix timestamp for a date with zero hours / minutes, and then adding time of day as seconds gives wrong result.

snippet from my code
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        echo ("\n<!-- before hms ".strftime ("%b %d %Y %H:%M:%S", $dateAndTime)."-->\n");
        $dateAndTime += 7200;
        echo ("\n<!-- before DST ".strftime ("%b %d %Y %H:%M:%S", $dateAndTime)."-->\n");
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example outputs

timestamp before arithmetic
<!-- before hms Oct 07 2001 00:00:00-->

after adding 7199 (ie 01:59)
<!-- before DST Oct 07 2001 01:59:59-->

after adding 7200 (ie 02:00)
<!-- before DST Oct 07 2001 03:00:00-->

only seems to happen for this transition between 1:59 and 2:00 , after that the 1 hour error is consistent.