How do I Convert Java Timestamps to a Specific Time Zone?

Handling Calendar Time Zones in Java

When dealing with timestamps in Java, it's crucial to consider time zones to ensure accurate date and time conversions. This becomes particularly important when integrating with web services that expect time parameters to be in a specific time zone, such as GMT.

A common scenario arises when timestamps are obtained from user input, and the user's time zone is unknown. To resolve this issue, we need to convert the user's local time into the desired time zone, such as GMT.

The provided code snippet exhibits a misunderstanding about the default time zone of timestamps. Timestamps are not inherently in GMT by default. They typically represent the current time in the system's local time zone.

To convert a timestamp into a Java calendar object in a specific time zone, a method called convertTimestampToJavaCalendar is utilized. However, the issue with this method arises when trying to set the calendar's time zone to GMT. Modifying the calendar's time zone does not automatically update the underlying date.

The solution to this problem lies in the convertToGmt method, which takes a calendar object and converts its date and offset to a new calendar object in the GMT time zone. Here's a step-by-step explanation of how it works:

1. Obtain the Date object representing the current time in the input calendar's time zone.
2. Determine the offset from GMT for the input calendar at the current time.
3. Create a new calendar object in the GMT time zone with the same date as the input calendar.
4. Adjust the new calendar by adding the offset in milliseconds from the input calendar.

This approach ensures that the resulting calendar object represents the correct time in the GMT time zone. For instance, if the input time is "12:09:05 EDT," the converted GMT time will be "08:09:05 GMT."

It's worth noting that this method works for situations where the web service deals with calendar parameters. Alternatively, if the web service expects time parameters in terms of milliseconds from the epoch, one can bypass the time zone conversion by simply obtaining the milliseconds from the input calendar.

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