Thanks for reporting! 👍

The bug is reproducible and looks to be tightly related to TimeZones and DTS. The one used in Paraguay has a DTS change at midnight 10-01 which is ambiguous to DateFormatter and nil is returned. Even similar examples from StackOverflow reference Paraguay timezone with this particular bug.

DateFormatter:

private var periodDataQueryDateFormatter: DateFormatter {
    let df = DateFormatter()
    df.locale = Locale(identifier: "en_US_POSIX")
    df.dateFormat = "yyyy-MM-dd"
    return df
}

po df.timeZone = TimeZone(identifier: "America/Asuncion")
po df.date(from: "2023-10-01")

nil

po df.timeZone = TimeZone(identifier: "Europe/Vilnius")
po df.date(from: "2023-10-01")

▿ Optional<Date>
  ▿ some : 2023-09-30 21:00:00 +0000
    - timeIntervalSinceReferenceDate : 717800400.

Solution

In documentation Apple suggests to set the timezone to UTC:

When working with fixed format dates, such as RFC 3339, you set the dateFormat property to specify a format string. For most fixed formats, you should also set the locale property to a POSIX locale ("en_US_POSIX"), and set the timeZone property to UTC.

RFC3339DateFormatter.timeZone = TimeZone(secondsFromGMT: 0)

However, this is not a straightforward solution, since in Stats we're working with the dates that are in site's timezone.