You can set the index_format variable to include all manner of different details about each message. In particular, you probably want the %d format string, which inserts the date formatted according to the value of date_format, or one of the other date format strings, such as %{fmt}, %[fmt], etc. As an example, here is my default index_format setting:
set 'index_format="%4C %Z %{%b %d} %-15.15L (%4l) %s"'
Take a look at the documentation for more details on what you can configure, and what the extended date format strings represent.