Wednesday, April 25, 2007

DST implementations in Operating Systems and Java

I often wondered how operating systems knew about Day light saving offsets. Recently USA decided to change its DST settings and it immediately crossed my mind as to how Operating Systems and Applications would react to this.

Some basics first - In what format does a OS store date/time information ? All machines keep their clocks in synch with the GMT using Network Time Protocol. Then the machine calculates it local time using the local time zone settings that are stored on the machine.
Local time is always computed as = GMT + timezone offset + DST offset

But where does the OS get information about DST. All DST information is stored in a binary database called ZoneInfo database or TimeZone DB. On Solaris this database can be found at "/usr/share/lib/zoneinfo"

In Java, the JRE picks up DST information from the Timezone database stored at "%JRE_HOME%/lib/zi"

Tip: In Java, U can change the default timezone that the application uses by setting the user.timezone property
e.g. java -Duser.timezone=Area/City
java -Duser.timezone=GMT[+5.30]

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