What's the difference between Instant and LocalDateTime?

java

time-management

java-8

best-practices

When you need to timestamp your logs or events in UTC, you use Instant like a boss:

Instant instant = Instant.now();  // Captures current "smoking gun" moment in UTC

But when you are designing a calendar app and couldn't care less about timezones, you go LocalDateTime all the way:

LocalDateTime localDateTime = LocalDateTime.now();  // Gives you the "my time, my rules" date and time

So, go Instant when getting lost in timezones is not an option and LocalDateTime when your app laughs in the face of time zones.

Pick the right Class

So you need to decide between Instant and LocalDateTime. Let's break it down:

Logging Events: Your software is playing detective and recording timestamps for posterity. That's when Instant gets called into action.
Handling User Interface: Your app is handling birthdays or local events. It's LocalDateTime time! Timezone? We don't need no stinking timezone!

Each class has its own way of dealing with timezones:

Timezone-agnostic: LocalDateTime and timezones are like oil and water. You get a LocalDateTime but no time zone, or offset from UTC.
Timezone-picky: Instant lives and breathes Coordinated Universal Time (UTC). It's all about precision, baby!

Say you want to serialize these timestamps or store them in a database:

Instant: Perfect for keeping a record in your databases. It's a point-in-time you can point at, universally!
LocalDateTime: Watch out! Serialization could be tricky due to timezone ambiguity.

Switching between Instant and LocalDateTime, consider:

Switching Lanes: Need UTC in LocalDateTime? Pair it with a ZoneId and voila! You have a ZonedDateTime.
Comparison Shopping: Comparing LocalDateTime to Instant? Turn your LocalDateTime into the Hulk (ahem ZonedDateTime) with a time zone, first!

If you have to deal with code that's stuck in the past:

Backport Lifesavers: ThreeTen-Backport or ThreeTenABP for Android can teleport java.time features to prehistoric Java versions.
JDBC Synchronization: After JDBC 4.2, OffsetDateTime is the secret sauce for time conversions.

If you are dealing with timezones:

Keep your Timezone Database Buffed: For correct ZonedDateTime conversions, update the tz database regularly.
Instant for the History Books: Log historical records in Instant for global consistency across timezones.

java.time plays well with others:

ISO-8601: Stick by the community-approved ISO-8601 standards for cross-technology harmonization.
Tech Pairing: java.time is reliable when dealing with databases, web APIs, and interchange formats like JSON.