Improve scheduled reminder UI for selection of event specific date or time before event

To set a scheduled reminder for an event, you can either set it to a specific date or n hours, days, etc before the event. However, the UI to set these two options is confusing. If you set a date and n hours before, the n hours before is ignored. The user must click the X beside the date to remove it in order to make the second option work, which is non-standard and confusing (I had a staff member save a reminder multiple times who couldn't figure out why the n hours before option was disappearing).

To make this clearer to users, I suggest adding radio buttons in front of the two options and only using the selected option.