FilmCalc.jsx is a palette to find running time from footage length or vice versa.
To use put the script into your scripts directory and run as per usual.
Use the radio buttons to set your film format, and set your framerate in the textbox.
Then either key in a running time or a footage length (any of the measures of feet/meters/frames will do)
Then if you have set a running time for example, click the footage length button and it will calculate, or to calculate running time set a length and then click running time.