serialize method
Implementation
String serialize() {
final buffer = StringBuffer();
buffer.write('FREQ=${frequency.name.toUpperCase()};INTERVAL=$interval');
if (until != null) {
buffer.write(';UNTIL=${formatDateTime(until!)}');
}
if (byDay != null && byDay!.isNotEmpty) {
buffer.write(';BYDAY=${byDay!.join(',')}');
}
return buffer.toString();
}