If the sellers do not accept your offer exactly as presented (no changes of any kind) within the time limits you've given, your offer is automatically rejected. Your commitment to the offer expires the moment the time period is up, or the sellers reject it, or you withdraw it. You can demand your deposit back and can now feel free to walk away from the property.
It's important to remember two things here: First, you can withdraw your offer at any time up until the sellers have accepted it. For example, you give the sellers until 10 p.m. to accept. But at 6 p.m. you discover a house that you like a whole lot better. You want to get out of the original offer you made, so you can buy the second house. You call your Hawaii agent and are told the sellers are considering your offer and probably will sign within the hour. You tell your agent that the offer is withdraw immediately and you want your deposit back. In theory, you may withdraw your offer at any time until you are informed of the sellers' written acceptance.
Second, the sellers cannot both accept and modify your' offer. Too often I've heard agents call their buyers and say, "Congratulations, you've bought a home! I'll be by for you to initial a couple of minor changes the sellers have made in the purchase agreement."
No way! If the sellers change anything at all, you're not committed to your offer, you haven't bought the property, there's been no deal made. The only way you're committed to your offer is if the sellers accept it as you signed it with no changes of any kind (within the time frame you've given for acceptance). The minute they make changes, it becomes a counteroffer. In other words, they've rejected your offer and are now countering with an alternative. You may feel free to accept or reject (or yourself counter) their counteroffer.
It's important to understand that this applies no matter how trivial appearing their change may be. For example, your offer may state that the deal is to close and you are to be given possession on the 25th of the month. The sellers agree to your price and terms; however, they change the offer to state that the deal is to close on the 30th, so they don't have to pay an additional 5 days interest (or rent) when they move to their new home.
It's now a counteroffer. You can simply say, "Nope," and walk away.
Of course, the date may make no difference to you and you'll be happy to accept. On the other hand, maybe now you'll think about the whole deal and decide you really don't want that property after all. (It's something the sellers have to consider seriously, before countering an offer.)