Need to make your footage duration fit your composition? You can let After Effects figure out how much stretch you really need. These two techniques are useful when you need to lengthen or shorten a shot.
To maintain your layer's out-point, you can stretch toward the beginning of the Composition. Press Cmd+Shift+Comma (Ctrl+Shift+Comma) to Time Stretch the In- point of a Layer.
To maintain your layer's in-point, you can stretch toward the end of the Composition. Press Cmd+Option+Comma (Ctrl+Alt+Comma) to Time Stretch the Out Point of a Layer.
When using either method, be certain to turn on Frame Blending. Enable it globally in the Composition and on the Time Stretched Layer.