Hi! I visited my fiance in the US in august using the visa waiver from the UK. I changed my flights by 4 days after I had arrived (still within the 90 days) and there was no issue coming back last weekend. So long as you don't overstay, it should be good.
You can also check your I-94W online and it will tell you the date you need to be out by (https://i94.cbp.dhs.gov/I94/#/home - if you click on the 'already visiting option' of the four, and input the info, it will give you a date).
They more than likely wanted to check the return ticket to ensure it was booked, and was within the 90 days! Enjoy your time together!
