Ok. That's fair.
Presumably, you're already using the signup date to calculate their 14 days.
Use the same signup date to calculate 180 days. There are a few different ways php can do that, depending on your version. (The php manual should give you all you need on that.)
If today is less than the signup+180, then reject. If not, then allow them to sign up again.
The other issue you need to consider is whether you create a new record for each sign-up, or whether you just update the signup date if the record exists. I'd do the latter, but I'd include a field in the file that counts the number of times they've signed up for the trial.