Getting timestamp of activation plugin time

You’ll have to use get- and add_option inside a register_activation_hook.

<?php
register_activation_hook(__FILE__,'save_activation_timestamp');
function save_activation_timestamp()
{
add_option("myplugin_activation_timestamp",mktime());
}
?>

You can find an entire tutorial on that at CodeYoghurt.
It even mentions your case as an example.