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 most commonly used tags in the fmt tag library is the <fmt:formatDate> tag, which is used for formatting date values.

The <fmt:formatDate> tag accepts a date value as input and formats it according to the specified pattern. The syntax of the tag is as follows:

<fmt:formatDate value="dateValue" pattern="dateFormatPattern" />

where dateValue is the date to be formatted and dateFormatPattern is the pattern used to format the date. The pattern attribute specifies the format of the output string, using a combination of letters and symbols that represent different parts of the date and time.

For example, to format the current date and time in the format "yyyy-MM-dd HH:mm:ss", you would use the following tag:

<fmt:formatDate value="${now}" pattern="yyyy-MM-dd HH:mm:ss" />

Here, ${now} is an expression that evaluates to the current date and time.

The output of the tag would be a string representing the current date and time in the specified format.

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.