How to convert a LocalDate to an Instant?

java

localdate

instant

timezone

byNikita Barsukov·Aug 16, 2024

Effortlessly convert a LocalDate to an Instant:

LocalDate date = LocalDate.now(); // ah, the present...good times!
Instant instant = date.atStartOfDay().atZone(ZoneId.systemDefault()).toInstant(); // Bam! We're in the future...sorta

This command attaches the default start time of day (midnight), ties it to a specific time zone, and, like a magic trick, gives you an Instant.

Time zone specifics in conversion

When converting a LocalDate (a plain date) to an Instant (a specific point on the time-line), we need a time value and a time-zone attached:

Choose a time of day—LocalTime.MIN for midnight, or whenever you feel most awake.
Specify a time zone. Use ZoneId such as ZoneId.systemDefault() or ZoneOffset.UTC for consistency.

Neglecting to assign the time zone could misrepresent your Instant—sounds scarier than it is, but it may ruin your afternoon.

Try these different time-zones on for size

LocalDate localDate = LocalDate.now();

// Using system's default time zone
Instant instantDefault = localDate.atStartOfDay(ZoneId.systemDefault()).toInstant();

// Using UTC for those global thinkers
Instant instantUTC = localDate.atStartOfDay(ZoneOffset.UTC).toInstant();

Your choice of ZoneId should fit the context of your application; if your mind is set on ZoneOffset.UTC, go for it!

Bypassing common conversion missteps

Trying Instant.from() with LocalDate:

Instant.from() and LocalDate aren't friends—they'll throw a DateTimeException due to LocalDate's inability to tell time (literally).

Using LocalDateTime as a stepping stone

A handy practice is to transition from LocalDate to LocalDateTime first, then to Instant. It's like giving LocalDate a shiny new watch before it becomes an Instant.

LocalDateTime localDateTime = LocalDateTime.of(date, LocalTime.MIN); // Spruce up the date with time
Instant instant = localDateTime.toInstant(ZoneOffset.UTC); // Fear not time-travelers, we're just specifying a time zone!

This method is your VIP pass to the start of the day of the given LocalDate, translated into UTC time.

Time and tide wait for none - Additional considerations

Beware of Daylight Saving Time

If you're dealing with time zones that observe Daylight Saving Time (DST), using ZoneId.systemDefault() could be like trying to hit a moving target.