You can also figure out the day in your head, with some simple manipulations. No need to be a math whiz. The key is that in terms of days of the week, your 180 day mark is two days before your check-in day. So if you check in on a Saturday, your window will open on a Thursday. If you check in on Wednesday, your window opens on a Monday. So first step is to figure out which day of the week you'll be making your reservations.
Next step is to take your check-in date, and back up six months. Then go forward two or three days, because six months is a little more than 180 days. This will have you really close, but not spot on. So from there, find the nearest day that falls on the correct day of the week. And now you're good to go. (The description sounds a lot more complicated than it really is. )