I need a way of only filtering data that is over 18 months old. Does anyone know which formula to use and how to use it,
There are a number of possibilities on how you might want to deal with this.
All of them are covered, with examples, in the embedded Help files. You could start with the "Date Function" entry assuming you have a date to work with or can generate one from the data available.
There are several other functions relating to Dats which would also be worth reading through. It is just possible that you input uses Julian Dates or some other form of date recording and there are sections covering options beyond regular dates.
The usual simple selection would be to 'adjust' the current date by a certain number of days. IN a simple case that would simply be a fixed number (say -548 days to pick a date for 18 months ago) but your needs may be more specific and more complex than that.