Improve yyyy-MM-dd date formatter to support dates happening at DST

[staskus]

Description

Fixes wordpress-mobile/WordPress-iOS#22859

We need to pass yyyy-MM-dd date to API when fetching stats/visits and other similar endpoints. However, in some cases, date conversions to date string fails.

The bug is related to TimeZone and DTS. For example, Paraguay has a DTS change at midnight 2023-10-01 which is ambiguous to DateFormatter and nil is returned.

Date formatter we use:

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

Result for "America/Asuncion" time zone:

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

Result for "Europe/Vilnius" time zone:

po df.timeZone = TimeZone(identifier: "Europe/Vilnius")
po df.date(from: "2023-10-01") // result <Date> 2023-09-30 21:00:00 +0000

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 dates that are in a different time zone.

Solution

Created RFC339NoTimeDateFormatter that sets:

df.locale = Locale(identifier: "en_US_POSIX")
df.dateFormat = "yyyy-MM-dd"
df.timeZone = TimeZone(secondsFromGMT: 0)

but also recalculates the data to the current time zone after conversion. Therefore, the dates remain unchanged but conversions also succeed. I added RFC339NoTimeDateFormatterTests as an example.

Testing Details

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• I have considered if this change warrants release notes and have added them to the appropriate section in the CHANGELOG.md if necessary.