Did you check if UInt64(total - id) * 1_000_000_000 statement overflows?

If so you could try something like this: let (value, overflowOccurred) = interval.multipliedReportingOverflow(by: 1_000_000_000) and then check overflowOccurred value

I've tried try await Task.sleep(nanoseconds: Int64.max) and it completes immediately. At the same time documentation doesn't state which max value could be passed to this method.