How to alert on number of days from a string date

[mellis]

I have a userparmeter setup that is returning a string that is a date. My boss now want to setup an alert that will let us know if that date is under 180 days. I would like to use a preprocessing on a dependent item so as to reduce the network payload. But even a trigger would work.

Anyone have an idea how to do date math off this string value?