Build your own date calculation. The problem you are experiencing is because Unices use 32bit numbers, and keep track of time based on the number of seconds since Jan 1, 1970 00:00. So using a 32bit number the address space for Unix time overflows in 2037. This will never change until we get to 64bit operating systems.
But you can write your own date convertion function that just takes in some values and some math to do the work. Something like:
function my_date_conv($org_date, $math) {
}
Figure out some way that you will understand to use the $math variable to do the work you want done and get to it.
