
If you are trying to change your timeline display units in your After Effects timeline you may find that it is difficult to change and not obvious. The current-time display appears in the upper-left corner of the Timeline panel and at the bottom of the Layer, Composition, and Footage panels. By default, After Effects displays time in
Society of Motion Picture and Television Engineers (SMPTE) timecode: hours, minutes, seconds, and frames. You can change to another system of time display, such as frames, or feet and frames of 16mm or 35mm film. Here are the differences:
Timecode Base:
Displays time as timecode, using the frame rate that you specify as the timecode base. Auto uses the rounded frame rate of the footage item or composition. If an item doesn’t have timecode (such as an audio file), After Effects uses a default value (30 fps for English and Japanese versions of After Effects, or 25 fps for French, German, Spanish, and Italian versions) or the last non-auto value you specified in the Project Settings dialog box. You can also specify that After Effects use a specific frame rate.
Frames:
Displays frame number instead of time. Use this setting for convenience when doing work that you are integrating with a frame-based format, like Flash.
Feet + Frames: Displays number of feet of film, plus frames for fractional feet, for 16mm or 35mm film. Numbering starts at the frame number that you specify with the Start Numbering Frames at value.
(From Adobe After Effects CS3 Help)