Hi - You'll first want to make sure in your rules you are setting your currentTime field to now() when the user submits - so that it has the time when the button is clicked.
To get just the time, you need to use substring-after - it would be something like substring-after(currentTime, "T") - IP date time is ISO, has a T between the date and the time....
I guess you could also do something like currentTime < concat(today(), "T17:00:00") - you'll need to experiment a bit. Oh, and you could just set your currentTime field to a substring-after(now(), "T"), so it does just have the time....