Conditional Formatting with dates

I'm trying to get dates within a column on a table highlighted depending on their value.

I know how to do this using today's date, but I am having issues in using a separate set date as the parameter.

For example;

I want all values in a table in =K7:K72 that are more than 30 days older than a value set in A3 to be highlighted red.

This should be quite easy, but every time I enter anything the whole column changes colour when this is incorrect.

Cheers in advance