The JSTL (JavaServer Pages Standard Tag Library) fmt tag library provides a set of tags for formatting and internationalizing data in JSP (JavaServer Pages) pages. One of the tags in the fmt tag library is the <fmt:setTimeZone> tag, which is used to set the time zone to use for formatting dates and times.

The <fmt:setTimeZone> tag is used to set the time zone to use for formatting dates and times. The time zone determines the offset from Coordinated Universal Time (UTC) to use when displaying dates and times. The tag should be placed at the top of the JSP page, before any data is sent to the server. The syntax of the tag is as follows:

<fmt:setTimeZone value="timeZoneValue" [scope="page|request|session|application"] />

where timeZoneValue is the value of the time zone to use, and scope is the scope of the attribute that stores the time zone (optional).

For example, to set the time zone to use for formatting dates and times to Pacific Standard Time (PST), you would use the following tag:

<fmt:setTimeZone value="America/Los_Angeles" />

This tag sets the time zone to use for formatting dates and times to PST.

You can also specify the scope of the attribute that stores the time zone. If no scope is specified, the attribute is stored in the page scope. For example, to store the time zone in the session scope, you would use the following tag:

<fmt:setTimeZone value="America/Los_Angeles" scope="session" />

This tag sets the time zone to use for formatting dates and times to PST and stores it in the session scope.

Note that the fmt tag library must be imported at the beginning of the JSP page using the following tag:

<%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt" %>

This imports the fmt tag library and assigns it a prefix of fmt.