How to convert date time input string with optional time into UNIX time using java.time

There are multiple problems in your code, let me try listing one by one

1. You are always parsing the input string into LocalDate, even it is representing with time

LocalDate date = LocalDate.parse("2016-06-21-10-19-22", formatter); //2016-06-21 LocalDate date = LocalDate.parse("2016-06-21", formatter); //2016-06-21 

2. After parsing the input string into LocalDate, you are always converting it into ZoneDateTime with starting time of the day which is 00:00:00

 ZonedDateTime resultado = date.atStartOfDay(ZoneId.of("UTC")); //2016-06-21T00:00Z[UTC] 

3. And finally you are converting same ZonedDateTime into Instant irrespective of time from input, which is why you are getting same result always

Instant i = resultado.toInstant().getEpochSecond(); //1466467200 

The best to achieve this problem will be using DateTimeFormatter with optional time representation in formatter, and using parseBest

DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd[-HH-mm-ss]"); TemporalAccessor dt = parser.parseBest(dateTimeString, LocalDateTime::from, LocalDate::from); if (dt instanceof LocalDateTime) { (LocalDateTime) dt).atOffset(ZoneOffset.UTC).toInstant(); } else { (LocalDate) dt).atStartOfDay(ZoneOffset.UTC).toInstant(); }