Repeat is supported only if the start date is a date field (not date-time field)

Repeat is supported only if the start date is a date field (not date-time field)

I am setting up a Schedule to send out a reminder a year after a membership form has been filled in.

I have a System field set up within Column Properties. When I try to set up a Yearly repeat  Schedule  I get this error:

Repeat is supported only if the start date is a date field (not date-time field) 

How can I change this System  date-time field to just a  date field?

Thanks