In the rule settings, select the Shift end from the current moment checkbox. Then, set the period to 30 days.
This will set the rule to end 30 days from the current time.
If rounding is not enabled, time slots will become available gradually throughout the day. To make the entire day available at once, select Round up to the beginning of the day. In this case, you will need to increase the shift by 1 day. For this example, set the period to 31 days.
If you want managers to be able to make reservations without these restrictions, go to Account parameters. Enable the Disable dynamic time boundaries checkbox.
Check the result in the widget: