If you just need the information on demand, the Get Info plugin is great:
If you want to display it in a file, you can install the Dataview plugin and add this snippet:
This page was last modified on `$= dv.current().file.mtime.toFormat("MMMM d, yyyy")` at `$= dv.current().file.mtime.toFormat("h:mm a")`.
You will need to enable inline javascript queries in the Dataview options.
Get Info Plugin
Dataview plugin