How do I convert java.util.Date to java.time\'s Instant, OffsetDateTime, or ZonedDateTime?

Convert java.util.Date to java.time's Instant, OffsetDateTime, or ZonedDateTime

As we migrate towards the modern java.time framework, it's essential to know how to convert legacy java.util.Date objects into the appropriate java.time types. Here's a rundown of the equivalencies:

java.util.Date to Instant

Both represent a moment in UTC, so the conversion is straightforward:

<code class="java">Instant instant = myUtilDate.toInstant();

java.util.Date myUtilDate = java.util.Date.from(instant);</code>

java.util.Date to java.time's OffsetDateTime or ZonedDateTime

Since these types incorporate time zone information, extracting the zone from the legacy Date is necessary:

<code class="java">// If the legacy date is a GregorianCalendar (which can hold time zone info)
if (myUtilCalendar instanceof GregorianCalendar) {
    GregorianCalendar gregCal = (GregorianCalendar) myUtilCalendar;
    ZonedDateTime zdt = gregCal.toZonedDateTime(); // ZonedDateTime with time zone

    java.util.Calendar myUtilCalendar = java.util.GregorianCalendar.from(zdt);
}</code>

Additional Conversion Mappings

Legacy Type	java.time Equivalent	Additional Notes
java.util.Calendar	Instant	Converts to the start of the day in UTC
java.util.GregorianCalendar	ZonedDateTime	Retains time zone information
java.util.LocalDate	ZonedDateTime	Requires a time zone to determine the date
java.util.LocalTime	Instant	Converts to the start of the day in UTC
java.util.LocalDateTime	ZonedDateTime	Requires a time zone to determine the date and time

Important Considerations

• Converting from a java.util.Date to a java.time type may result in loss of precision as nanosecond resolution is not supported by the legacy classes.
• When converting to OffsetDateTime or ZonedDateTime, the time zone information must be preserved to ensure correct interpretation.
• It's highly recommended to use java.time types in new code for enhanced precision and consistency.

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