Thread: Timezones and time/timestamp values in FE/BE protocol
I'm working on a problem in the JDBC driver that's related to timezones. How does PostgreSQL handle timezones in the FE/BE protocol exactly? When a client sends a time or timestamp value to the server via the FE/BE protocol, should that be: 1) a value in the client's timezone? 2) a value in the server's timezone? 3) a value in a common frame of reference (GMT/UTC)? 4) any value with an explicit timezone? And how should a time or timestamp value returned by the server be interpreted in the client interface? And how does this all depend on the timezone setting of the server? Regards, René Pijlman <rene@lab.applinet.nl>
Rene, Since the FE/BE protocol deals only with string representations of values, the protocol doesn't have too much to do with it directly. It is what happens on the client and server sides that is important here. Under the covers the server stores all timestamp values as GMT. When a select statement queries one the value is converted to the session timezone and formated to a string that includes the timezone offset used (i.e. 2001-09-09 14:24:35.12-08 which the database had stored as 2001-09-09 22:24:35.12 GMT). The client then needs to handle this accordingly and convert to a different timezone if desired. On an insert or update the client and server are essentially doing the opposite. The client converts the timestamp value to a string and then the server converts that string to GMT for storage. If the client does not pass the timezone offset (i.e. 2001-09-09 14:24:35.12 instead of 2001-09-09 14:24:35.12-08) then the server needs to guess the timezone and will use the session timezone. Now when it comes to the JDBC code this is what happens. (Since you didn't state what specific problem you where having I will give a general overview). When the JDBC driver connects to the server it does one thing timestamp related. It does a 'set datestyle to "ISO"' so that the client and the server both know how the strings are formated. I don't know what the session timezone defaults to, but it really shouldn't matter since the server always sends the timezone offset as part of the string representation of the timestamp value. Therefore the JDBC client can always figure out how to convert the string to a Java Timestamp object. On the insert/update opperation the JDBC client converts the Timestamp object to GMT (see the logic in setTimestamp() of PreparedStatement) and then builds the string to send to the server as the formated date/time plus the timezone offset used (GMT in this case). Thus it does something that looks like: "2001-09-09 14:24:35.12" + "+00". When the server gets this string it has all the information it needs to convert to GMT for storage (it actually doesn't need to do anything since the value is clearly already in GMT). I hope this helps to answer your questions. If you could post a bit more about the issue you are having I might be able to be more specific. thanks, --Barry Rene Pijlman wrote: > I'm working on a problem in the JDBC driver that's related to > timezones. > > How does PostgreSQL handle timezones in the FE/BE protocol > exactly? > > When a client sends a time or timestamp value to the server via > the FE/BE protocol, should that be: > 1) a value in the client's timezone? > 2) a value in the server's timezone? > 3) a value in a common frame of reference (GMT/UTC)? > 4) any value with an explicit timezone? > > And how should a time or timestamp value returned by the server > be interpreted in the client interface? > > And how does this all depend on the timezone setting of the > server? > > Regards, > René Pijlman <rene@lab.applinet.nl> > > ---------------------------(end of broadcast)--------------------------- > TIP 3: if posting/reading through Usenet, please send an appropriate > subscribe-nomail command to majordomo@postgresql.org so that your > message can get through to the mailing list cleanly > >
On Sun, 09 Sep 2001 13:38:52 -0700, you wrote:
[...]
Thanks for your explanation. This helps a lot.
>If you could post a bit more about the issue you are having
>I might be able to be more specific.
I'm looking at the 4 remaining failures of our own JDBC test
suite. They all have to do with timestamps and times, and they
are all caused by a 1 hour shift between the expected value and
the actual value. I run both the backend and the JVM on the same
Linux test server.
Its located in Amsterdam, The Netherlands, Central European
Daylight Savings Time (CETDST, UTC+2, GMT+2). I always thought I
was in CET=GMT+1, but now the offset is 2, because of daylight
saving time (whoever invented that should be #!$^&). Perhaps I
should go live in Greenwich, they don't seem to have daylight
saving time overthere.
In psql I see:
show timezone;
NOTICE: Time zone is unset
Here is some detailed information about the failures. I'm
refering to line numbers in 7.2 current CVS:
TimeTest.java revision 1.1
TimestampTest.java revision 1.2
1) TimeTest.java:89
getHours(t) expected 1, actual 0
t.toString() returns the expected "01:02:03", but this is
because java.sql.Time.toString() converts to the JVM's timezone.
2) TimeTest.java:96
getHours(t) expected 23, actual 0
t.toString returns "00:59:59"
3) TimestampTest.java:115
Expected: getTimestamp(1970,6,2,8,13,0) returns "1970-06-02
08:13:00.0"
Actual: t.toString() returns "1970-06-02 09:13:00.0"
4) TimestampTest.java:115 (second time around)
Expected: getTimestamp(1970,6,2,8,13,0) returns "1970-06-02
08:13:00.0"
Actual: t.toString() returns "1970-06-02 07:13:00.0"
My first impression is that in all cases a timezone shift is
applied in only one direction (store vs. retrieve). The cause
might also be a problem with daylight saving time, there are
some comments about that in TimestampTest.java.
Up till now I've managed without a graphical debugger, but to
get a good feel for what's happening between the test code and
the wire I think it'll be easier to setup JBuilder with the
driver and step through the code.
But now its almost bedtime in my timezone, and you never know
with these mailing lists. Sometimes the solution is in your
inbox when you wake up :-)
Regards,
René Pijlman <rene@lab.applinet.nl>
On Sun, 09 Sep 2001 23:43:36 +0200, I wrote: >I'm looking at the 4 remaining failures of our own JDBC test >suite. They all have to do with timestamps and times FYI, Liam mailed me that he will soon post a patch for this. Regards, René Pijlman <rene@lab.applinet.nl>