March 14th not 86400 seconds long?

Question

In my web application, I have users input a date in a simple textbox. That input (after being sanitized, of course), is run through strtotime(), and 86399 is added to it, to make that timestamp the end of the day written (11:59:59). This is for due date purposes (so if the date passes, the application raises a flag)

For the days I tested, it worked...

January 5th saved as january 5th, at the end of the day.

March 13th saved as March 13th

March 15th saved as March 15th

March 14th, for whatever reason, saved itself as March 15th.

Is March 14th mysteriously a couple seconds short or something??

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Update: Thanks to oezi for the solution - worked like a charm. Code as requested:

Old code:

if ($_POST['dateto'] != '') {
    $dateto = strtotime(mysql_real_escape_string($_POST['dateto'])) + 86399;
}

New code:

# Offset to "end of day"
list($y,$m,$d) = explode('-',date("Y-m-d",strtotime($_POST['dateto'])));
$d++;
$dateto = strtotime($y . '-' . $m . '-' . $d) - 1;

Answer 1

March 14, 2010 is the day Daylight Saving Time begins in the United States. So if you're doing math in the local time zone, March 14 is only 23 hours long.

Answer 2

I would assume because this is the beginning of daylight savings time