Subtract 30 days from promted date

Hi experts,

I try to make a universe filter which give me 30 days before day prompted. On where condition I write something like this:

to_char(start_time) between (to_date(@prompt('Day','D',,,)) - interval '30' day) and to_date(@prompt('Day','D',,,)))

I have less days tan I expect. Do you know what is wrong on the sentence? do you have other way to do it?

Thank you