When skipped frames occur, their timing information is stored under the UserVars field of the data file. The meaning of each column in SkippedFrameTimeInfo is like the following.
SkippedFrameTimeInfo(:,1) - trialtime that the frame was supposed to be presented
SkippedFrameTimeInfo(:,2) - trialtime that the frame was actually presented
SkippedFrameTimeInfo(:,3) - The number of frames skipped
SkippedFrameTimeInfo(:,4) - Frame length (msec)
SkippedFrameTimeInfo(:,5:end) - Additional time information