Is it Java best practice to store dates as longs in your database?

Question

My reason for doing so is that dates stored as date objects in whatever database tend to be written in a specific format, which may greatly differ from what you need to present to the user on the front-end. I also think it's especially helpful if your application is pulling info from different types of data stores. A good example would be the difference between a MongoDB and SQL date object.

However, I don't know whether this is recommended practice. Should I keep storing dates as longs (time in milliseconds) or as date objects?

Answer 1

Since this question is tagged with MongoDB: MongoDB does not store dates in String or what not, they actually store it as a long ( http://www.mongodb.org/display/DOCS/Dates ):

A BSON Date value stores the number of milliseconds since the Unix epoch (Jan 1, 1970) as a 64-bit integer. v2.0+ : this number is signed so dates before 1970 are stored as a negative numbers.

Since MongoDB has no immediate plans to utilise the complex date handling functions (like getting only year for querying etc) that SQL has within standard querying there is no real downside, it might infact reduce the size of your indexes and storage.

There is one thing to take into consideration here, the aggregation framework: http://docs.mongodb.org/manual/reference/aggregation/#date-operators there are weird and wonderful things you can only with the supported BSON date type in MongoDB, however, as to whether this matters to you depends upon your queries.

Do you see yourself as needing the aggregation frameworks functions? Or would housing the extra object overhead be a pain?

My personal opinion is that the BSON date type is such a small object that to store a document without it would be determental to the entire system and its future compatibility for no apparent reason. So, yes, I would use the BSON date type rather than a long and I would consider it good practice to do so.

Answer 2

It depends on various aspects. When using the standard "seconds since epoch", and someone uses only integer precision, their dates are limited to the 1970-2038 year range.

But there also is some precision issue. For example, unix time ignores leap seconds. Every day is defined to have the same number of seconds. So when computing time deltas between unix time, you do get some error.

But the more important thing is that you assume all your dates to be completely known, as your representation does not have the possibility to half only half-specified dates. In reality, there is a lot of events you do not know at a second (or even ms) precision. So it is a feature if a representation allows specifing e.g. only a day precision. Ideally, you would store dates with their precision information.

Furthermore, say you are building a calendar application. There is time, but there also is local time. Quite often, you need both information available. When scheduling overlaps, you of course can do this best in a synchronized time, so longs will be good here. If you however do also want to ensure you are not scheduling events outside of 9-20 h local time, you also always need to preserve timezone information. For anything that does span more than one location, you really need to include the time zone in your date representation. Assuming that you can just convert all dates you see to your current local time is quite naive.

Note that dates in SQL can lead to odd situations. One of my favorites is the following MySQL absurdity:

SELECT * FROM Dates WHERE date IS NULL AND date IS NOT NULL;

may return records that have the date 0000-00-00 00:00:00, although this violates the popular understanding of logic.

Answer 3

It very much depends on:

• What database you're using and its date/time support
• Your client needs (e.g. how happy are you to bank on the idea that you'll always be using Java)
• What information you're really trying to represent
• Your diagnostic tools

The third point is probably the most important. Think about what the values you're trying to store really mean. Even though you're clearly not using Noda Time, hopefully my user guide page on choosing which Noda Time type to use based on your input data may help you think about this clearly.

If you're only ever using Java, and your database doesn't have terribly good support for date/time types, and you're only trying to represent an "instant in time" (rather than, say, an instant in a particular time zone, or a local date/time with an offset, or just a local date/time, or just a date...), and you're comfortable writing diagnostic tools to convert your data into more human readable forms - then storing a long is reasonable. But that's a pretty long list of "if"s.

If you want to be able to perform date manipulation in the database - e.g. asking for all values which occur on the first day of the month - then you should probably use a date/time type, being careful around time zones. (My experience is that most databases are at least shocking badly documented when it comes to their date/time types.)

In general, you should use whatever type is able to meet all your requirement and is the most natural representation for that particular environment. So in a database which has a date/time type which doesn't give you issues when you interact with it (e.g. performing arbitrary time zone conversions in an unrequested way), use that type. It will make all kinds of things easier.

The advantage of using a more "primitive" representation (e.g. a 64 bit integer) is precisely that the database won't mess around with it. You're effectively hiding the meaning of the data from the databae, with all the normal pros and cons (mostly cons) of that approach.

Answer 4

IMHO , storing dates in DB will be best if you can use Strings. Hence avoid unnecessary data going up and down to server , if you don't need all the fields in Calendar. There is a lot of data is in Calendar and each instance of Calender is pretty heavy too.

So store it as String , with only required data and convert it back to Calendar , whenvever you need them and use them.

Answer 5

I dont think its a best practice to store dates as long because, that would mean that you would not be able to do any of the date specific queries. like :

where date between 

We also wont be able to get the date month of year from the table using sql queries easily.

It is better to use a single date format converter in the java layer and convert the date into that and use a single format throughout the application.

Answer 6

I can't speak for it in relation to MongoDB, but in SQL database, no, it's not best practice. That doesn't mean there might not be the occasional use case, but "best practice," no.

Store them as dates, retrieve them as dates. Your best bet is to set up your database to store them as UTC (loosely, "GMT") so that the data is portable and you can use different local times as appropriate (for instance, if the database is used by geographically diverse users), and handle any conversions from UTC to local time in the application layer (e.g., via Calendar or a third-party date library).

Storing dates as numbers means your database is hard to report against, run ad-hoc queries against, etc. I made that mistake once, it's not one I'll repeat without a really good reason. :-)