note : LocalDateTime is

1. LocalDateTime converted to custom time format string

public static String getDateTimeAsString(LocalDateTime localDateTime, String format) {
    DateTimeFormatter formatter = DateTimeFormatter.ofPattern(format);
    return localDateTime.format(formatter);
}

2. Convert long type timestamp to LocalDateTime

public static LocalDateTime getDateTimeOfTimestamp(long timestamp) {
    Instant instant = Instant.ofEpochMilli(timestamp);
    ZoneId zone = ZoneId.systemDefault();
    return LocalDateTime.ofInstant(instant, zone);
}

3. LocalDateTime to long type timestamp

public static long getTimestampOfDateTime(LocalDateTime localDateTime) {
    ZoneId zone = ZoneId.systemDefault();
    Instant instant = localDateTime.atZone(zone).toInstant();
    return instant.toEpochMilli();
}

4. LocalDateTime

converts a time string to a custom time format

public static LocalDateTime parseStringToDateTime(String time, String format) {
    DateTimeFormatter df = DateTimeFormatter.ofPattern(format);
    return LocalDateTime.parse(time, df);
}