let's say your refference data is
$data="2000-08-15";
so what do you need is to transform a string into a format that php understands as a date.
hopefully strftime() function parses this string into a timestamp (the number of seconds that passed since the unix era -1970-01-01).
then you can actually make whatever operations you want with strftime() which can even use dummy words like "-1 month" (see examples)
it is important when you use strftime to use setlocale() function;
look at these examples
setlocale(LC_TIME, "en_EN");
$data1=strftime("%Y-%m-%d",strtotime("$data -1 month"));
$data3=strftime("%Y-%m-%d",strtotime("$data -3 months"));
$data6=strftime("%Y-%m-%d",strtotime("$data -6 months"));
$data12=strftime("%Y-%m-%d",strtotime("$data -1 year"));
i hope it will help