How to find date values which are more than 10 days old

Question

I am using Java and iterating over D.B. column which in return gives me date and time string as shown below:

String dateTime = resultSet.getSeries().get(0).getValues().get(0).get(0);

If I iterate on this resultset I am getting dateTime values in format as shown below.

2017-07-20T19:21:37.987792408Z
2017-04-24T22:04:26.808753375Z
2017-08-14T22:22:40.340772396Z
2017-06-24T22:24:32.422544491Z
2017-07-31T22:27:05.893368615Z

Out of these records, how can I compare date string with "current" date object and discard those values which are more than 10 days old?

I tried

Date date = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssZ").parse(s);

That didn't work. Any other idea?

Edit: I am using Java7 and using InfluxDB that does not provide sysdate column while querying. So I have to come up with solution using Java.

Answer 1

java.time

Retrieve date-time values as date-time objects, not strings.

The Instant class represents a moment on the timeline in UTC with a resolution of nanoseconds.

Instant instant = myResultSet.getObject( … , Instant.class ) ;

Compare to ten days before the current moment.

Instant now = Instant.now() ;
Instant tenDaysAgo = now.minus( 10 , ChronoUnit.DAYS ) ;
Boolean prior = instant.isBefore( tenDaysAgo ) ;

You may not want to base your "ten days ago" on UTC. If not, apply a time zone to get a ZonedDateTime and LocalDate. This has been covered many times so search Stack Overflow. Think about if "days" means (a) chunks of 24-hours or (b) calendar dates to you.

FYI, those strings happen to be in standard ISO 8601 format. The T separates the date portion from time portion. The Z is short for Zulu and means UTC. This Instant class uses the same format in its toString method.

SQL

Generally, you should do such comparison work in the database as part of the SQL query rather than in your Java app. The database is highly tuned for this work; your app is not.

String sql = "SELECT * FROM tbl_ WHERE when_ < ? " ;
… make your PreparedStatement
myPreparedStatement.setObject( 1 , tenDaysAgo ) ;
… execute

Answer 2

Assuming dates is a list of strings that represent dates such as in your example, with Java 8 ZonedDateTime and DateTimeFormatter APIs you can simply do:

dates = dates.stream()
        .map(ZonedDateTime::parse)
        .filter(z -> z.isAfter(ZonedDateTime.now().minusDays(10L)))
        .map(Zon‌​edDateTime::toString‌​)
        .collect(Collectors.toList());

System.out.println(dates);  // [2017-08-14T22:22:40.340772396Z]

UPDATE
@Basil and @shmosel raised a valid point in their comments below, the check, the way it's implemented right now, checks now() against each one of the strings (this now() is different for any such comparison).

If we want to check the same specific point in time, against all the strings we should "freeze" now(), a nice way to do it (thanks @shmosel for the suggestion) would be to create a single instance of now() and using its method reference:

ZonedDateTim‌​e.now().minusDays(10‌​)::isAfter

or:

ZonedDateTime tenDaysAgo = ZonedDateTime.now().minusDays(10L);
dates = dates.stream()
        .map(ZonedDateTime::parse)
        .filter(z -> z.isAfter(tenDaysAgo))
        .map(Zon‌​edDateTime::toString)
        .collect(Collectors.toList());