Actually the default time isn't now but the next hour. As I type it is 15h03. The offered reminder date & time is today at 16h00.
However, if I first select a date and time for the task, then I am offered the option to set a reminder at the Due Time, 30 mins, 1 hour or 1 day before.
So start by setting the task date/time and you'll get what you are proposing.