tl;dr
ZonedDateTime.now( // Capture the current moment as seen through the wall-clock time used by the people of a certain region (a time zone).
ZoneId.of( "Pacific/Auckland" ) // Specify a time zone using proper name, `continent/region`, never 3-4 pseudo-codes such as `PST`, `EST`, `IST`.
)
.minusDays( 10 ) // Go back in time ten days, adjusting for time-of-day as need be.
.toLocalDate() // Extract a date-only value.
.atStartOfDay( // Determine the first moment of that date in a certain time zone. Not always 00:00:00.
ZoneId.of( "Pacific/Auckland" )
)
.toString() // Generate a String in standard ISO 8601 format.
2018-03-23T00:00+13:00[Pacific/Auckland]
Avoid legacy date-time classes.
You are using troublesome old date-time classes supplanted years ago by the java.time classes.
java.time
Instead of Calendar
, use ZonedDateTime
to represent a moment on the timeline with a wall-clock time used by people of a certain region (time zone).
A time zone is crucial in determining a date. For any given moment, the date varies around the globe by zone. For example, a few minutes after midnight in Paris France is a new day while still “yesterday” in Montréal Québec.
If no time zone is specified, the JVM implicitly applies its current default time zone. That default may change at any moment, so your results may vary. Better to specify your desired/expected time zone explicitly as an argument.
Specify a proper time zone name in the format of continent/region
, such as America/Montreal
, Africa/Casablanca
, or Pacific/Auckland
. Never use the 3-4 letter abbreviation such as EST
or IST
as they are not true time zones, not standardized, and not even unique(!).
ZoneId z = ZoneId.of( "Africa/Tunis" ) ;
ZonedDateTime zdt = ZonedDateTime.now( z ) ; // Capture the current moment.
If you want to use the JVM’s current default time zone, ask for it and pass as an argument. If omitted, the JVM’s current default is applied implicitly. Better to be explicit, as the default may be changed at any moment during runtime by any code in any thread of any app within the JVM.
ZoneId z = ZoneId.systemDefault() ; // Get JVM’s current default time zone.
ZonedDateTime zdt = ZonedDateTime.now( z ) ; // Capture the current moment.
Math
Specify a span of time unattached to the timeline of years-months-days using Period
.
Period p = Period.ofDays( 10 ) ;
Go back in time. Anomalies such as Daylight Saving Time (DST) are handled automatically, adjusting time-of-day as need be.
ZonedDateTime zdtTenDaysAgo = zdt.minus( p ) ;
“Midnight” is a trick concept, ambiguous and amorphous. Instead, focus on the idea of “first moment of the day”.
Always let java.time determine the first moment. Do not assume the day starts at 00:00:00. Anomalies such as DST mean the day may start at another time such as 01:00:00. To get that first moment, extract a date-only LocalDate
object. Specify a time zone to determine when that date began in that place.
LocalDate ldTenDaysAgo = zdtTenDaysAgo.toLocalDate() ;
ZonedDateTime zdtTenDaysAgoStartOfDay = ldTenDaysAgo.atStartOfDay( z ) ;
If you want to view the same moment as UTC, extract a Instant
.
Instant instant = zdtTenDaysAgoStartOfDay.toInstant() ;
About java.time
The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date
, Calendar
, & SimpleDateFormat
.
The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.
To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.
You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.*
classes.
Where to obtain the java.time classes?
The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval
, YearWeek
, YearQuarter
, and more.