Adding date to logback.xml

Have tried to search for it but I keep on getting ...IS_UNDEFINED as my file name. I simply want to append the current date to a log file. Any simple example for the logback.xml file?

This is my latest try:

<Properties>
  <property name="filePattern">log_${date:yyyy-MM-dd}.log</property>
</Properties>
<appender name="FILE" class="ch.qos.logback.core.FileAppender">
  <file>${application.home:-.}/logs/${filePattern}</file>
  <encoder>
    <pattern>%date [%level] from %logger in %thread - %message%n%xException</pattern>
  </encoder>
</appender>