Psychtoolbox

It is necessary to log onset timing of stimuli for first level analysis, and important to be aware of how time is recorded.

• http://psychtoolbox.org/docs/Screen-Flip

  Flip (optionally) returns a high-precision estimate of the system time (in seconds)
   when the actual flip has happened in the return argument ‘VBLTimestamp’.
   ....
  ‘FlipTimestamp’ is a timestamp taken at the end of Flip’s execution.
   Use the difference between FlipTimestamp and VBLTimestamp to get an estimate of 
   how long  Flips execution takes. [in what units!?]

• PTBs Timestamp FAQ

  FlipTimestamp, which is not much more than a GetSecs taken at end of Flip, can and
   often will be earlier than StimulusOnsetTimestamp, as flip processing usually already
    completes while the display is still in VBLANK, preparing for display of the new stimulus 
    frame, that's why StimulusOnsetTimestamp == VBLTimestamp > FlipTimestamp.
  

w = Screen('OpenWindow', 0, [0 0 0], [0 0 800 600]); 
for i=1:3; 
   [vts(i,1), vts(i,2)] = Screen('Flip',w); 
   vts(i,3) = GetSecs(); 
end;
sca
format longG
vts,

On linux (+ octave), GetSecs() as well as Flip's VBLTimestamp and StimulusOnsetTime are in epoch seconds.

vts =

       1738935521.40479        1738935521.40479       1738935521.405101
       1738935521.421465       1738935521.421465      1738935521.421387
       1738935521.438137       1738935521.438137      1738935521.438507

On windows, these numbers or clock ticks since start up. Similar to EPrime Task Presentation Software.

[foran] windows output