Handling time zone offset transition and daylight savings time with Joda

Question

I am trying to parse datetime strings and create Joda DateTime objects.

My data comes from a legacy database that stores datetime strings without specifying the timezone/offset. Although the timezone/offset of the datetime strings is not stored it is a business rule of the legacy system that all datetimes are stored in Eastern Time. Unfortunately I do not have the authority to update the way in which the legacy DB stores datetime strings.

Thus, I parse the datetime strings using JODA's "US/Eastern" time zone.

This approach throws an illegalInstance exception when the dateTime string falls within the hour that "disappears" when daylight savings is switched on.

I've created the following example code to demonstrate this behaviour and to show my proposed workaround.

public class FooBar {
public static final DateTimeZone EST = DateTimeZone.forID("EST");
public static final DateTimeZone EASTERN = DateTimeZone.forID("US/Eastern");

public static final DateTimeFormatter EST_FORMATTER = DateTimeFormat.forPattern("yyyy-MM-dd HH:mm:ss.SSS").withZone(EST);
public static final DateTimeFormatter EASTERN_FORMATTER = DateTimeFormat.forPattern("yyyy-MM-dd HH:mm:ss.SSS").withZone(EASTERN);


public static void main(String[] args) {
    final String[] listOfDateTimeStrings = {"2014-03-09 02:00:00.000", "2014-03-08 02:00:00.000"}; 

    System.out.println(" *********** 1st attempt  *********** ");
    for (String dateTimeString: listOfDateTimeStrings){
        try{
            final DateTime dateTime = DateTime.parse(dateTimeString, EASTERN_FORMATTER);
            System.out.println(dateTime);       
        }
        catch(Exception e){
            System.out.println(e.getMessage());
        }
    }

    System.out.println(" *********** 2nd attempt  *********** ");
    for (String dateTimeString: listOfDateTimeStrings){
        try{
            final DateTime dateTime = DateTime.parse(dateTimeString, EST_FORMATTER);
            System.out.println(dateTime);       
        }
        catch(Exception e){
            System.out.println(e.getMessage());
        }
    }

    System.out.println(" *********** 3rd attempt  *********** ");
    for (String dateTimeString: listOfDateTimeStrings){
        try{
            DateTime dateTime = DateTime.parse(dateTimeString, EST_FORMATTER);
            dateTime = dateTime.withZone(EASTERN);
            System.out.println(dateTime);       
        }
        catch(Exception e){
            System.out.println(e.getMessage());
        }
    }       

}

}

Output produced:

 *********** 1st attempt  *********** 
Cannot parse "2014-03-09 02:00:00.000": Illegal instant due to time zone offset transition (America/New_York)
2014-03-08T02:00:00.000-05:00
 *********** 2nd attempt  *********** 
2014-03-09T02:00:00.000-05:00
2014-03-08T02:00:00.000-05:00
 *********** 3rd attempt  *********** 
2014-03-09T03:00:00.000-04:00
2014-03-08T02:00:00.000-05:00

In the "3rd attempt" I get the expected result: the first datetime has an offset of -04:00. as it falls within the first hour of DST for 2015. The second timestamp has an offset of -05:00 as it falls outside of DST.

Is is safe to do this:

DateTime dateTime = DateTime.parse(dateTimeString, A_FORMATTER_WITH_TIME_ZONE_A);
dateTime = dateTime.withZone(TIME_ZONE_B);

I've tested this code with a few different combinations of datetime strings and time zones (and so far it works for all the test cases), but I was wondering if anyone with more Joda experience can see anything wrong/dangerous in this approach.

Or alternatively: is there a better way of doing handling the time zone offset transition with Joda?

+1 Nicely written question, with one minor exception: "Is the code ... safe to use?" What do you mean by "safe"? If you test it with sufficient edge cases, does it produce the expected and desired behavior? You'll have to describe in more detail what you're asking specifically. — Jim Garrison
@JimGarrison thanks for pointers; I've updated the end of the question, hopefully it's more specific now. — YB -Abeokuta
When you say "all datetimes are stored in EST", do you literally mean that they are all stored in UTC-5 - Eastern Standard Time, or do you mean that they are stored in Eastern Time, which is UTC-5 (EST) for part of the year and UTC-4 (EDT) for the other part? Because you also said, "when daylight savings is switched on in the EST timezone", I'm thinking you mean "Eastern Time", not "EST". (Daylight saving does not apply during standard time.) — Matt Johnson-Pint
@MattJohnson You are correct, I meant to say "all datetimes are stored in Eastern Time". I've edited the question. Apologies for the confusion. — YB -Abeokuta

Jim Garrison Jim Garrison · Accepted Answer · 2016-01-05T17:26:27

Your question boils down to

I have to cope with timestamp values that are not technically valid and interpret them consistently

What you do with them depends totally on your requirements. If you are writing the software for an employer, the owner of the project must be the one to make that decision. You as the developer do not yet have the authority to decide what to do with invalid input if that hasn't been specified by the designer/architect.

I suggest you go back to the project owner/manager, inform them of the problem (input contains date/time stamps that don't actually exist) and get them to make the decision on how to handle them.

Handling time zone offset transition and daylight savings time with Joda

2 Answers