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Relying on the Default TimeZone

Relying on the Default TimeZone

Calendar calendar = new GregorianCalendar();calendar.setTime(date);calendar.set(Calendar.HOUR_OF_DAY, 0);calendar.set(Calendar.MINUTE, 0);calendar.set(Calendar.SECOND, 0);Date startOfDay = calendar.getTime();

The code above calculates the start of the day (0h00). The first mistake is the missing millisecond field of the Calendar, but a major mistake is not setting the TimeZone of the Calendar object, as a result, the Calendar will use the default time zone. In a Desktop application this might be fine, but in server-side code this is a big problem. As an example, 0h00 in Tokyo is in a very different moment than in New York, so the developer should check which time zone is relevant for this computation.

Calendar calendar = new GregorianCalendar(user.getTimeZone());calendar.setTime(date);calendar.set(Calendar.HOUR_OF_DAY, 0);calendar.set(Calendar.MINUTE, 0);calendar.set(Calendar.SECOND, 0);calendar.set(Calendar.MILLISECOND, 0);Date startOfDay = calendar.getTime();
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