PHP getTimestamp not working when adding 100 years

Question

I need to build a calculator which adds 100years and out puts as timestamp. but i can't get it to work over X number or years?

***WORKS

$date = new DateTime('1980-02-15');
$date->modify('+15 year');
echo $date->getTimestamp();

***DOESNT WORK

$date = new DateTime('1980-02-15');
$date->modify('+105 year');
echo $date->getTimestamp();