Function format_datetime 

pub fn format_datetime(
    dt: &RosettaDateTime,
    fmt: &str,
    lang: Option<&LanguageData>,
) -> String

Format a RosettaDateTime using a strftime-style format string.

Supported specifiers:

• %Y — 4-digit year
• %m — 2-digit month (01–12)
• %d — 2-digit day (01–31)
• %H — 2-digit hour in 24h (00–23)
• %M — 2-digit minute (00–59)
• %S — 2-digit second (00–59)
• %z — UTC offset (+0800)
• %Z — UTC offset with colon (+08:00)
• %A — Full weekday name (from language data, or English default)
• %a — Abbreviated weekday name
• %B — Full month name
• %b — Abbreviated month name
• %p — AM/PM
• %% — Literal %