Why is my Java code converting date and time to CST incorrectly, and how can I fix it?

Converting Date and Time to a Specific Timezone in Java

This Java program aims to convert the current system date and time to a specific timezone. However, the provided code yields incorrect results, showing a time discrepancy when converting from India to Central Standard Time (CST).

Understanding Timezone Conversion

Timezones are regional areas that use the same standard time. When calculating the equivalent time in another timezone, you must adjust for the difference in time zone offsets and daylight saving time (DST) settings.

Analyzing the Code

The code relies on the SimpleDateFormat and Calendar classes to manipulate date and time values. It attempts to convert the current system time to CST using the following steps:

1. Retrieves the current date and time using Calendar.
2. Parses the date and time to a String using SimpleDateFormat.
3. Obtains the CST timezone using TimeZone and sets it as the timezone for the SimpleDateFormat object.
4. Parses the date and time string using the CST timezone.
5. Prints the converted date and time.

Addressing the Discrepancy

The discrepancy arises because the code does not properly adjust for the time zone offset and DST settings. The provided code only adjusts for the raw time zone offset but does not account for DST.

Solution

To address this issue, the code should adjust for both the raw offset and DST savings. The following corrected code demonstrates the adjustments:

<code class="java">Calendar calendar = Calendar.getInstance();
TimeZone fromTimeZone = calendar.getTimeZone();
TimeZone toTimeZone = TimeZone.getTimeZone("CST");

// Adjust for raw time zone offset
calendar.add(Calendar.MILLISECOND, fromTimeZone.getRawOffset() * -1);

// Adjust for daylight saving time
if (fromTimeZone.inDaylightTime(calendar.getTime())) {
    calendar.add(Calendar.MILLISECOND, calendar.getTimeZone().getDSTSavings() * -1);
}

// Adjust to target time zone
calendar.add(Calendar.MILLISECOND, toTimeZone.getRawOffset());
if (toTimeZone.inDaylightTime(calendar.getTime())) {
    calendar.add(Calendar.MILLISECOND, toTimeZone.getDSTSavings());
}

System.out.println(calendar.getTime());</code>

By adding these adjustments, the code will accurately convert the date and time to the specified timezone, including DST adjustments.

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