It depends on where you’re getting your recurrences from? I don’t recall any Apple-specific recurrence mechanisms on the level of calendars like you’re describing. But eventually if you have date objects, you’ll want to use DateComponents to extract the kind of data you’re describing (I don’t know what RRules are)
How you get a DateComponents object is below. See the docs for how to extract the individual months, days etc.

let date = Date()
var calendar = Locale.current.calendar
var dateComponents = calendar.dateComponents(Set<Calendar.Component>(), from: date)

To convert an iOS date to the RRULE format commonly used in recurring events, you’ll need to extract the necessary components from the date object. Start by getting the year, month, day, and time information from the iOS date. Then, format these components into the RRULE syntax, following the RFC5545 specification. Utilize libraries or custom functions to handle the conversion efficiently, ensuring that the resulting RRULE string accurately represents the recurrence pattern based on the iOS date.

If you’re looking to convert iOS dates and recurrence patterns to RRULE format, there are a few approaches you can take in Swift. You can either manually construct the RRULE string based on your recurrence logic, or use libraries like SwiftRRule, which handle conversion from Date and recurrence options (daily, weekly, monthly, etc.) to the proper RRULE string.

For example, if you want a weekly recurrence on weekdays, the RRULE string would look like:

"FREQ=WEEKLY;BYDAY=MO,TU,WE,TH,FR"

Using a library makes it easier to handle complex recurrence rules, especially when sending them to the backend. This is especially useful if you’re working on a production-grade app—you want to avoid edge-case bugs with date and timezone conversions.

Leveraging these libraries can save time and help ensure your iOS app properly communicates recurrence rules to your backend services without reinventing the wheel.