customDateHeaderText property
If dateFormat, dateLocale and/or timeFormat is not enough to customize date headers in your case, use this to return an arbitrary string based on a DateTime of a particular message. Can be helpful to return "Today" if DateTime is today. IMPORTANT: this will replace all default date headers, so you must handle all cases yourself, like for example today, yesterday and before. Or you can just return the same date header for any message.
Implementation
final String Function(DateTime)? customDateHeaderText;