Like Choroba has already pointed out, use a module to handle the date/time calculations for you. Use that module to evaluate the products_last_modified value, something he also pointed out. Then, it looks like you could update your if statement to include another clause, something along the lines of:

# update when price/stock conditions met
if (($data->{'price'} <= $product_price && $data->{'quantity'} > 0).
|| ($product_quantity == 0 && $data->{'quantity'} > 0) 
|| ($products_last_modified <= 12))
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