Function parse_scheduled_time_datetime 

pub fn parse_scheduled_time_datetime(
    extensions: &HashMap<String, String>,
) -> Result<Option<DateTime<Utc>>>

Parse the scheduled time from extensions into a DateTime<Utc>.

Returns Ok(None) if the extension key is absent, Ok(Some(datetime)) on success, or Err if the value is malformed. Millis that produce an out-of-range timestamp are also rejected.