We have been using similar language for several years and it was modified to refer to the intent to proceed when TRID came along. It has passed several FDIC exams.
The 15 days is the number of days we chose. You can used 10, 15, 20, 30 or whatever you are comfortable with. Your action taken date will be the letter's expiration date or the date you close the file for incompleteness.
The opinions expressed are mine and they are not to be taken as legal advice.