Flexibility of NS e-tickets

Hi. I'm travelling to the Netherlands in late September. I have the NS train app and buying single tickets is reasonably straightforward. You pick a date and a time and select for one or two options. But I've seen confusing information online that seems to suggest that even if you are asked to choose a time, you can actually travel that day at any time. I'm doubtful of that but -- does anyone know? I would like that flexibility. There are options if you buy an OV-chipkaart but I will be only making 2 or 3 trips and I don't want to commit to or pay for a service I won't really use. Thank you.