Unfortunately you cannot add or subtract frames in the timecode display of AE. It would be a nice feature request.
I almost always work in Frames in AE, not with timecode, but this works in either. If you hold down the shift key and the Cmnd/Ctrl key and press the left arrow key once you'll move back 10 frames. Do this 4 times and then release the shift key and press the Ctrl/Cmnd + right arrow 4 times and you've 36 frames from your original starting position.
from After Effects Help:
"To go forward a specific period of time or number of frames, click the current-time display, and then enter the plus sign ( + ) followed by the timecode or number of frames to advance. For example, enter +20 to go forward 20 frames or 1:00 to go forward one second. Precede the value by the minus sign ( - ) to go backward. For example, enter +-20 to go backward 20 frames or +-1:00 to go backward one second."
Todd is right as usual, I was unclear. What I meant to say is that you cannot go to the Time display when it says 0:00:01:15 then move to the end and type - 30 frames and have AE do the math like it does with things like position and rotation. Unfortunately, if you are at 0:00:1:15 and you simply type -4 you will end up at -0:00:00:04 frames not 0:00:01:01 as you would expect. You have to type +-4 to make it work. Just remember the + sign before you type any negative numbers and you'll be OK.
I knew it had to be simple, not necessarily logical, but simple.
This is one of the more hidden and confusing parts of our user interface, in my opinion.