Unix timestamps in Java

Get the current Unix timestamp

long seconds = Instant.now().getEpochSecond(); // 1700000000
long millis = System.currentTimeMillis();      // 1700000000000

Convert a timestamp to a date-time

Instant instant = Instant.ofEpochSecond(1700000000L);
ZonedDateTime tokyo = instant.atZone(ZoneId.of("Asia/Tokyo"));
// 2023-11-15T07:13:20+09:00[Asia/Tokyo]

An Instant is zone-free by design; attach a ZoneId only when you need a wall-clock rendering.

Convert a date-time to a timestamp

long seconds = zonedDateTime.toInstant().getEpochSecond();
long millis = zonedDateTime.toInstant().toEpochMilli();

Parse an ISO 8601 string

Instant instant = Instant.parse("2023-11-14T22:13:20Z");
LocalDateTime local = LocalDateTime.parse("2023-11-14T22:13:20"); // no zone!

Instant.parse requires the Z or an offset. A LocalDateTime is a wall clock with no zone; it cannot become a timestamp until you say which zone it was in.