WordPress at a glance
function is not described

wp_cache_get_last_changed() WP 4.7.0

Get last changed date for the specified cache group.

No Hooks.

Return

String. $last_changed UNIX timestamp with microseconds representing when the group was last changed.

Usage

wp_cache_get_last_changed( $group );
$group(string) (required)
Where the cache contents are grouped.

Code of wp cache get last changed: wp-includes/functions.php VER 5.1.1

<?php
function wp_cache_get_last_changed( $group ) {
	$last_changed = wp_cache_get( 'last_changed', $group );

	if ( ! $last_changed ) {
		$last_changed = microtime();
		wp_cache_set( 'last_changed', $last_changed, $group );
	}

	return $last_changed;
}