Most of the time I've found that those flexible date searches only allow a period of one or two weeks. I wish I could see every date in a month! Other than cetain holidays I'll want to go home for, I'm pretty flexible.
I haven't used yahoo travel for yonks, because I've found other sites that in my opinion find me a better fare. But as I recall, you put in the date range you're looking for (say Jan-May), it lists available fares, you click the fare you want, then it brings up a calendar of what dates that particular fare is available. In my experience, though, even though yahoo says the fare is available on such and such a date, it may not actually be available. So if you select that date, you get a message saying that there are no seats available at that fare and you need to select either a different date or a different fare. (The is the US yahoo travel site...the UK site works a bit differently.)
So it doesn't matter when you come back, as long as you do? So if I book a round trip ticket in the fall, I can just use the return at any time? Are there any restrictions? Do you sites like priceline and hotwire allow you to do that or is it the airline?
It depends on the kind of ticket you buy. Full-fare tickets are unrestricted, meaning you can change travel dates with no penalty, but they are also very expensive. I usually end up with 'restricted' tickets, sometimes meaning that they cannot be changed, but usually meaning there's a hefty penalty for change. If you're sure that you can travel on the dates you've chosen, a restricted ticket is going to be MUCH less expensive.