How to get DayOfWeek from an Instant.now()

Question

Assuming the following code...

Instant x = Instant.now();

How do I get day of week from x?

Related https://stackoverflow.com/questions/33353190/obtain-day-of-the-week-from-epoch — Naman, May 29 '19 at 15:53
An `Instant` is an absolute time value, the day of week would depend on where on Earth you are. — Jim Garrison, May 29 '19 at 16:14
@JimGarrison - as I understand it, any instance of Instant is assumed Zulu or UTC, which implies Greenwich. From this could we not derive the day of the week? — barrypicker, May 29 '19 at 16:17
No, as @techtabu mentions in his/her answer, you have to explicitly provide a timezone. — Jim Garrison, May 29 '19 at 16:19

techtabu · Accepted Answer · 2019-05-29T15:54:55.863

30

You have to convert it to ZonedDateTime

Instant.now().atZone(ZoneId.systemDefault()).getDayOfWeek()

edited May 29 '19 at 15:54

answered May 29 '19 at 15:49

techtabu

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`atZone(ZoneId.of("UTC"))` is more likely to give the intended result. – Oleg May 29 '19 at 15:58
@Oleg - thank you. You are correct, I do not want local time, which is why I am using Instant. – barrypicker May 29 '19 at 16:19
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An `OffsetDateTime` suffices and in the UTC case makes more sense. Also @Oleg – Ole V.V. May 29 '19 at 16:54
@Oleg is it really necessary to use a string there? Aren't there any constants? :\ – t3chb0t Jan 17 '21 at 13:53
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@t3chb0t https://stackoverflow.com/questions/44836984/is-there-a-reason-for-java-time-zoneid-not-including-an-enum-of-zoneids – Oleg Jan 17 '21 at 17:33

score 6 · Answer 2 · answered May 29 '19 at 18:26

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I have awarded points to techtabu, but I ended up using atOffset instead. Here is where I ended up...

int currentDayOfWeekValue = Instant.now().atOffset(ZoneOffset.UTC).getDayOfWeek().getValue();

I am amazed how difficult the Java8 datetime libraries are. There are so many variations of similar concepts...

Instant
LocalDate
LocalTime
LocalDateTime
OffsetDateTime
ZoneOffset
ZonedDateTime

Rhetorical questions:

Is Zulu and UTC the same or different?

What is the timezone associated with Instant.now() - the results suggest Zulu?

Why can't I manipulate an Instant object like a LocalDateTime - methods are similar but different?

How are ZonedDateTime and OffsetDateTime different - they seem to be addressing the same concept.

answered May 29 '19 at 18:26

barrypicker

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Zulu and UTC just names for are the same (nitpickers may say that Zulu is offset zero form UTC, so conceptually not exactly the same, but the time is the same). There is *no* timezone or offset associated with `Instant`. `ZonedDateTime` is a date and time in a time zone like America/Los_Angeles and will respect DST in your time zone when you add days or months to it. `OffsetDateTime` is a datetime with an offset like -08:00 and will always count a day as 24 hours. – Ole V.V. May 29 '19 at 18:44
`Instant` and `LocalDateTime` are both time zone neutral but quite different beasts. An `Instant` is what the class name says. You might also say a moment in time. Since time zone neutral it really hasn’t got a name (but its `toString` produces date and time in UTC). A `LocalDateTime` is a date and time of day without time zone or offset, so does *not* define an instant. – Ole V.V. May 29 '19 at 18:50
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Simpler: Either `OffsetDateTime.now(ZoneOffset.UTC).getDayOfWeek()` or for example `LocalDate.now(ZoneOffset.UTC).getDayOfWeek()`. – Ole V.V. May 29 '19 at 19:04
OffsetDateTime does not take into account the DST – Alexander Nov 26 '19 at 16:35
@Alexander - correct UTC does not observe daylight savings time (DST). This is what I want - UTC without daylight savings and without system default, and without local time interference. I simply want UTC in the most raw format and consistent not changing to arbitrary government rules like DST. In fact, in Oregon a ballot measure was introduced to abolish DST. Similar to California as I understand it. – barrypicker Nov 26 '19 at 19:10
Makes sense. Thanks for clarifying. – Alexander Nov 27 '19 at 08:35

score -3 · Answer 3 · answered May 29 '19 at 16:13

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Calendar.getInstance().get(Calendar.DAY_OF_WEEK);

will give you the same result

answered May 29 '19 at 16:13

kleopi

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- the old-fashioned and not recommended way. Tha `Calendar` class is long outdated and poorly designed, so when it is not asked about, please don't introduce it. Also this will give you dumb number that at least I can never remember how to interpret. The modern code gives an enum constant with an explanatory name. – Ole V.V. May 29 '19 at 16:56

How to get DayOfWeek from an Instant.now()

3 Answers3

Rhetorical questions: