Currently, Donation Goals require you to set an end point. Which, though good for say, fundraising nights, don't really allow you to set an infinite goal in any way.
Say you're using it as a goal for your tips, the goal being you getting a new microphone. The goal doesn't need an ending: It will end when you can afford a new microphone, and you don't have a time frame on that beyond after you get the goal money on it.
Not only is it simply ugly to say, put it a year ahead and have "365 days remaining" over the top of the goal, but it can be annoying to manage, as if you don't put the date far enough it will expire. It'd be very simple to just let you opt out of setting a goal end date, so you can actually use it for a long term goal that will be completed when you get to the goal - not when the time limit runs out.