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Sunday, April 8, 2018

Right, Wrong and Right again about TimeZones in FHIR

So I am back to dates again.  I forgot to check in the change for setting the servers default time zone, and almost all my unit tests failed on a build machine located outside of my home zone.  Dang it.  I KNEW that would happen.

So I made the fix (following the advice of reasonably smart people), and set the server's default time zone to UTC.  And my date tests are still failing.

Here's the failure:  There's a record that updated on 2017-06-26T17:51:45.233-07:00 (the source of that record is on the left coast, it doesn't matter where the server is).

And I query for it by asking for records updated on that date using _lastUpdated=2017-06-26.

What should happen?

Well, everyone I asked who answered without much thought say that these should match. But they don't.

Because 2017-06-26T17:51:45.233-07:00 is 2017-06-27T00:51:45.233Z in UTC.  And clearly 2017-06-26 is not the same date once you transform the timestamp to UTC, and therefore _lastUpdated=2017-06-26 fails.

The principle of least surprise should apply, and if we use a fixed time zone for the server, it surely doesn't.  Especially a case where the server time zone isn't one my users care about.

So, the conclusion I've come to is, when comparing two dates:

IF both specify precision, the comparison proceeds based on the time as reflected by each date, as it was specified.  There's no problem here.  If neither specifies a time zone, any time zone will do, as long as it is the same for both.

Here's the tricky bit: if A does not specify a precision but B does, the comparison should be based on a common time frame, either A specified a TimeZone and B didn't, or A didn't and B did.  In the presence of a time zone in A or B but NOT both, they should be compared using the same zone.  Otherwise, someone is going to get an unexpected result.

It's that odd duck case where the user asked for a particular date, the server has a specific date and time zone specified for the date (which may be different from where the server itself is located), and boom.

Otherwise, imagine what could happen.  Any test without a time zone is by necessity, imprecise.  I think it's better to specify what should happen in these cases quite specifically, so that time based comparisons don't surprise anyone.

Date has a legal function, time stamps a technical one.  Comparing a DateTime might be used for either case.  So, in my view, even though it seems weird, one has to consider that a DateTime comparison WILL behave differently when using 2017-06-27T00:00+00:00 as compared to 2017-06-27.  The questions are different.  The first is about time, the second about a date, and the two are DIFFERENT but related things.

Ah well.  Another day goes by.

     Keith

P.S. A similar problem will show up when comparing durations. Did it happen in the last two days or in the last 48 hours are two DIFFERENT questions, just as when I speak to my colleagues in India yesterday, today and tomorrow mean different things to each of us, depending on where we are in the world.  Let's not even talk about business days ...

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