PHP Figuring out a repeating event's future dates - Works only until 2037?

I am writing a calendar-based web application and am working on some code that will loop X amount of times and figure out the start/end dates & times of the future events. Each event has a start date/time and an end date/time The loop runs perfectly fine up until the year 2037, at which point the loop stop function properly. The conversion never takes place and reverts my dates back to 1969. Please see the code and output below. Can someone tell me why this is happening?

Live Example of Problem Behavior: http://codepad.org/h5ET3h2O

$date1 = strtotime('August 17 2011 8:00 AM');

for ($x=1; $x<=$repeatTimes; $x++){

    echo date('m/d/Y g:i A', $date1)."<br><br>".$date1 ." +". $repeatFreq." ".$repeatUnit. " = ";
    $date1 = strtotime(date('m/d/Y g:i A', $date1) . " +".$repeatFreq." ".$repeatUnit );
    echo $date1."<br /><br />";


}

Output

08/15/2011 8:00 AM
1313409600 +1 Year = 1345032000

08/15/2012 8:00 AM
1345032000 +1 Year = 1376568000

08/15/2013 8:00 AM
1376568000 +1 Year = 1408104000

//Some Left Out

08/15/2036 8:00 AM
2102414400 +1 Year = 2133950400

//Problem starts here

08/15/2037 8:00 AM
2133950400 +1 Year = 

12/31/1969 7:00 PM
+1 开发者_StackOverflow中文版Year = 31536000

12/31/1970 7:00 PM
31536000 +1 Year = 63072000

12/31/1971 7:00 PM
63072000 +1 Year = 94694400

12/31/1972 7:00 PM
94694400 +1 Year = 126230400

12/31/1973 7:00 PM
126230400 +1 Year = 157766400

12/31/1974 7:00 PM
157766400 +1 Year = 189302400

//Some Left Out

UPDATE

I was able to utilize the DateTime Class to get the date working properly.

for ($x=1; $x<=$repeatTimes; $x++){
    $date1->modify(" +".$repeatFreq." ".$repeatUnit);
    $sDates[]=$date1->format('m/d/Y g:i A');
}

http://en.wikipedia.org/wiki/Unix_time

You're hitting the max value of a 32-bit integer.

Time is measured as the number of seconds since January 1, 1970 UTC. You see 1969 because in your timezone, that's when this would occur. When the number of seconds overflows the 32-bit int, it loops back around.

You can confirm the max integer value with this: echo PHP_INT_MAX;.

If you want 64-bit integers, you need to use 64-bit hardware, and a 64-bit version of PHP (which usually these days, you have to compile yourself, as I don't believe official binaries are available on PHP.net yet).

There are also other options. See here: how to have 64 bit integer on PHP?

Edit: Per Maerlyn's suggestion, you should consider using the DateTime class built into recent versions of PHP. It should get you around this problem, as long as you use it consistently.