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Converting string to ‘Instant’

I am trying to convert datetime in a string to an Instant instance using Java 8 or a utilities package.

For example,

String requestTime = "04:30 PM, Sat 5/12/2018";

to

Instant reqInstant should result in 2018-05-12T20:30:00.000Z

reqString is in the America/Toronto time zone.

This is what I tried

 String strReqDelTime = "04:30 PM, Sat 5/12/2018";
 Date date = new SimpleDateFormat("hh:mm a, EEE MM/dd/yyyy").parse(requestTime);
 Instant reqInstant = date.toInstant();

The above code results in “2018-05-12T23:30:00Z”.

How can I do it?

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Answer

tl;dr

  • Fix your formatting pattern for unpadded month and day.
  • Use only java.time classes, never the legacy classes.

Contrived example:

LocalDateTime.parse(                   // Parse as an indeterminate `LocalDate`, devoid of time zone or offset-from-UTC. NOT a moment, NOT a point on the timeline.
    "04:30 PM, Sat 5/12/2018" ,        // This input uses a poor choice of format. Whenever possible, use standard ISO 8601 formats when exchanging date-time values as text. Conveniently, the java.time classes use the standard formats by default when parsing/generating strings.
    DateTimeFormatter.ofPattern("hh:mm a, EEE M/d/uuuu", Locale.US)  // Use single-character `M` & `d` when the number lacks a leading padded zero for single-digit values.
)                                      // Returns a `LocalDateTime` object.
.atZone(                               // Apply a zone to that unzoned `LocalDateTime`, giving it meaning, determining a point on the timeline.
    ZoneId.of("America/Toronto")       // Always specify a proper time zone with `Contintent/Region` format, never a 3-4 letter pseudo-zone such as `PST`, `CST`, or `IST`.
)                                      // Returns a `ZonedDateTime`. `toString` → 2018-05-12T16:30-04:00[America/Toronto].
.toInstant()                           // Extract a `Instant` object, always in UTC by definition.
.toString()                            // Generate a String in standard ISO 8601 format representing the value within this `Instant` object. Note that this string is *generated*, not *contained*.

2018-05-12T20:30:00Z

Use single-digit formatting pattern

You used MM in your formatting pattern, to mean any single-digit value (months January-September) will appear with a padded leading zero.

But your input lacks that padded leading zero. So use a single M.

Ditto for day-of-month I expect: d rather than dd.

Use only java.time

You are using troublesome flawed old date-time classes (Date & SimpleDateFormat) that were supplanted years ago by the java.time classes. The new classes entirely supplant the old. There isn’t any need to mix the legacy and modern.

LocalDateTime

Parse as a LocalDateTime because your input string lacks any indicator of time zone or offset-from-UTC. Such a value is not a moment, and it is not a point on the timeline. It is only a set of potential moments along a range of about 26-27 hours.

String input = "04:30 PM, Sat 5/12/2018";
DateTimeFormatter f = DateTimeFormatter.ofPattern("hh:mm a, EEE M/d/uuuu", Locale.US);  // Specify locale to determine human language and cultural norms used in translating that input string.
LocalDateTime ldt = LocalDateTime.parse(input, f);

ldt.toString(): 2018-05-12T16:30

ZonedDateTime

If you know for certain that input was intended to represent a moment using the wall-clock time used by the people of the Toronto Canada region, apply a ZoneId to get a ZonedDateTime object.

Assigning a time zone gives meaning to your unzoned LocalDateTime. Now we have a moment, a point on the timeline.

ZoneId z = ZoneId.of("America/Toronto");
ZonedDateTime zdt = ldt.atZone(z);  // Give meaning to that `LocalDateTime` by assigning the context of a particular time zone. Now we have a moment, a point on the timeline.

zdt.toString(): 2018-05-12T16:30-04:00[America/Toronto]

Instant

To see that same moment as UTC, extract an Instant. Same moment, different wall-clock time.

Instant instant = zdt.toInstant();

instant.toString(): 2018-05-12T20:30:00Z

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. There isn’t any need for strings or for java.sql.* classes.

Where can we 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.

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