I am using Drupal and I changed the way the timezone field is used in the users table. It used to contain the number of seconds difference. For example when my timezone was EST (America/Montreal), then this field would have -18000 as the value.
-5 * 60 * 60 == -18000
Now, this field contains the actual timezone id (America/Montreal or EST). This is much more precise and support daylight savings.
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