PDF (adobe.com)

Change units in the timeline ruler or time display

 Right-click the timeline ruler or time display, and select one of the following:
HMS
(Timeline ruler only) Shows hours, minutes, and seconds.

Decimal (HH:MM:SS:mmm)
Shows hours, minutes, seconds, and milliseconds.

Samples (HH:MM:SS:sample)
Shows hours, minutes, seconds, and audio samples.

Samples
Shows only audio samples. (This option is unavailable for multitrack files.)
To better understand samples, see Digitizing audio.

FPS options
Show frames-per-second for various film and video formats, including high-definition options ranging from 50–60 fps. DF and NDF indicate drop-frame and non-drop frame formats, respectively.

Feet + Frames options
(Time display only) Show location in traditional 16mm or 35mm film. You should display these units when editing a digital proxy in a film-based project.

Custom
Shows the custom time format specified in the Preferences dialog box. The default, 12 frames-per-second, matches the default in Adobe Flash Professional. If your Flash projects use a different frame rate, choose Edit Custom Time Format.