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Conversion rather than revamp

The idea is to refactor the project as soon as possible to store Date/DateTime/Time values into java.time classes

Depending on the size and complexity of your codebase, this may be quite risky.

A more conservative approach is to do all new programming in java.time. To interoperate with old code not yet updated to java.time, convert. You will find new conversion methods on the old classes. Look for to… & from… methods. These conversion methods provide complete coverage, allowing you to go back and forth.

Another word of caution when revamping code: Many/most programmers have a poor understanding of date-time handling. The topic is surprisingly tricky and complicated, the concepts slippery when first encountered. Our quotidian understanding of date-time actually works against us when dealing with the strictness of programming and databases. So beware of poor code, buggy code, that mishandles date-time. Each time you discover such faulty code, you will be opening up a barrel of trouble as reports may have been produced showing incorrect results, databases may store invalid data, etc.

You can search existing Stack Overflow questions and answers to learn more. Below are a few brief points to help guide you.

Moment versus Not-a-moment

Date values into LocalDate

Date and time values into LocalDateTime

Incorrect. Or incomplete, I should say.

I recommend you frame your thinking this way… There are two kinds of time tracking:

• Moment (definite)
• Not a moment (indefinite)

Moment

A moment is a specific point on the timeline. Moments are tracked in java.time as a count of whole seconds, plus a fractional second as a count of nanoseconds, since the epoch reference of first moment of 1970 in UTC. "in UTC" is short for "an offset from the temporal meridian of UTC of zero hours-minutes-seconds".

The basic class for representing a moment is java.time.Instant. Objects of this class represent a moment as seen in UTC, always in UTC. This class is the basic building-block of the jav.time framework. This class should be your first and last thought when dealing with moments. Use Instant to track a moment unless your business rules specifically involve a particular zone or offset. And java.util.Date is specifically replaced by java.time.Instant, with both representing a moment in UTC but with Instant having a finer resolution of nanoseconds over the java.util.Date resolution of milliseconds.

Two other classes track moments in java.time: OffsetDateTime & ZonedDateTime.

OffsetDateTime represents a moment as seen with an offset-from-UTC other than zero. An offset is merely a number of hours-minutes-seconds ahead or behind UTC. So people in Paris set the clocks on their wall one or two hours ahead of UTC, while people in Tokyo set their clocks nine hours ahead, whereas people in Nova Scotia Canada set their clocks three or four hours behind UTC.

A time zone is much more than a mere offset. A time zone is a named history of the past, present, and future changes to the offset used by the people of a particular region as decided by their politicians. A time zone has a name in the format of Continent/Region such as Europe/Paris & Asia/Tokyo.

To view a date and time-of-day through the wall-clock time of a particular region, use ZonedDateTime.

Note that you can easily and cleanly move back and forth between these three classes, Instant, OffsetDateTime, and ZonedDateTime via their to…, at…, and with…methods. They provide three ways of looking at the very same moment. Generally you use Instant in your business logic and data storage and data exchange, while using ZonedDateTime for presentation to the user.

Not-a-moment

On the other hand, we have "not a moment" tracking. These are the indefinite types.

The not-a-moment class causing the most confusion is LocalDateTime. This class represents a date with a time-of-day but lacking any concept of offset or time zone. So if I say noon next January 23rd 2024, LocalDateTime.of( 2024 , Month.JANUARY , 23 , 12 , 0 , 0 , 0 ), you have no way of knowing if I mean noon in Tokyo, noon in Toulouse, or noon in Toledo Ohio US — three very different moments several hours apart.

When in doubt, do not use LocalDateTime. Generally in programming business apps we care about moments, and so rarely use LocalDateTime.

The big exception, when we most often need LocalDateTime in business apps, is booking appointments, such as for the dentist. There we need to hold a LocalDateTime and time zone (ZoneId) separately rather than combining them into a ZonedDateTime. The reason is crucial: Politicians frequently change time zone rules, and they do so unpredictably and often with little forewarning. So that dentist appoint at 3 PM may occur an hour earlier, or an hour later, if the politicians there decide:

• To adopt Daylight Saving Time (DST).
• To abandon DST.
• To stay on DST year-round (the latest fad).
• That a different offset would bring business opportunities. Example: Iceland adopting an offset of zero.
• That a different offset would have political effects. Example: India instituting one single time zone across the broad sub-continent.
• To adjust their clock as a diplomatic move relative to a neighboring country. Example: North Korea changing their offset to match South Korea.
• To adopt the offset of an invading/occupying force.

All of these happen much more often than most people realize. Such changes break naïvely-written apps needlessly.

The other indefinite types include:

• LocalDate for a date-only value without a time-of-day and without any zone/offset.
• LocalTime for a time-only value without any date and without any zone/offset.

The LocalDate can be tricky in that many people are unaware that, for any given moment, the date varies around the globe by time zone. Right now is "tomorrow" in Tokyo Japan while simultaneously "yesterday" in Edmonton Canada.

Your code

LocalDateTime localDateTime = date.toInstant().atZone(ZoneId.systemDefualt()).toLocalDateTime();

That code has two problems.

• One is the inappropriate use of LocalDateTime. The java.util.Date class behind date var represents a moment with an offset of zero. You discard the crucial piece of information, the offset, when converting to LocalDateTime.
• Another problem is the use of ZoneId.systemDefualt(). This means the results vary depending on the JVM’s current default time zone. As discussed above, this means the date may vary, not just the time-of-day. The same code running on two different machines may produce two different dates. This may be what you want in your app, or this may be a rude awakening to an unaware programmer.

Your other piece of code also has problems.

public static Date getBeginOfTheYear(Date date){
    LocalDate localDate = date.toInstant().atZone(ZoneId.systemDefualt()).toLocalDate();
    LocalDate beginOfTheYear = localDate.withMonth(1).withDayOfMonth(1);
    return Date.from(beginOfTheYear.atStartOfDay().atZone(ZoneId.systemDefault()).toInstant());
}

First of all, be aware there are unfortunately two Date classes amongst the legacy date-time classes: java.util.Date for a moment, and java.sql.Date that pretends to represent a date-only but is actually a moment (in a horrendously poor class-design decision). I will assume your means java.util.Date.

Secondly, avoid these kinds of call-chains with this kind of code. Readability, debugging, and logging are all made more difficult. Write simple short lines instead when doing a series of conversions. Use comments on each line to justify the operation, and explain your motivation. And, avoid the use of var for the same reason; use explicit return types when doing such conversions.

The date.toInstant() is good. When encountering a java.util.Date, immediately convert to a Instant.

Again, the use of ZoneId.systemDefualt() means the results of your code vary by the whim of any sys-admin or user who is changing the default time zone. This might be the intent of the author of this code, but I doubt it.

The part beginOfTheYear.atStartOfDay() produces a LocalDateTime when you pass no arguments. So you are again discarding valuable info (offset) without gaining anything in return.

Another problem: Your code will not even compile. The java.util.Date.from method takes an Instant, not a LocalDateTime returned by your call beginOfTheYear.atStartOfDay().

To correctly get the first moment of the first of the year of a particular moment, you almost certainly would want the people deciding business rules to dictate a particular time zone.

ZoneId zTokyo = ZoneId.of( "Asia/Tokyo" ) ;

You should indeed convert the incoming java.util.Date to a Instant.

Instant instant = myJavaUtilDate.toInstant() ;

Then apply the zone.

ZonedDateTime zdt = instant.atZone( zTokyo ) ;

Extract year portion. As discussed above, the date may vary by time zone. Therefore, the year may vary depending on the zone you provided in the previous line of code above!

Year year = Year.from( zdt ) ;

Get first day of that year.

LocalDate firstOfYear = year.atDay( 1 ) ;

Let java.time determine the first moment of that day in that zone. Do not assume the start time is 00:00. Some dates in some zones start at another time such as 01:00.

Notice that in determining the first moment of the year, we pass the argument for ZoneId to atStartOfDay, in contrast to your code. The result here is a ZonedDateTime rather than the LocalDateTime in your code.

ZonedDateTime zdtFirstMomentOfTheYearInTokyo = firstOfYear.atStartOfDay( zTokyo ) ; 

Lastly, we convert to java.util.Date. In greenfield code, we would avoid this class like the Plague. But in your existing codebase, we must convert to interoperate with the parts of your old code not yet updated to java.time.

Instant instantFirstMomentOfTheYearInTokyo = zdtFirstMomentOfTheYearInTokyo.toInstant(); 
java.util.Date d = java.util.Date.from( instantFirstMomentOfTheYearInTokyo ) ;

MongoDB

You said:

During test I immediately find an issue, a localDateTime variable with value of 2023-10-07 12:00:00 was saved on MongoDB as 2023-10-07 10:00:00 This is not a problem as long as when the data is fetched back into java, the value is back to 2023-10-07 12:00:00, but this is not happening so it is a big problem.

Too much to unpack there, with too little detail from you.

I suggest you post another Question specifically on that issue. Provide sufficient detail, along with example code, to make a diagnosis.