# Timing issue

**URL:** <https://psychtoolbox.discourse.group/t/timing-issue/1472>\
**Category:** Uncategorized\
**Created:** [July 22, 2008, 1:04am UTC](https://psychtoolbox.discourse.group/t/timing-issue/1472 "2008-07-22T01:04:16Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![Allen\_Ingling](https://avatars.discourse-cdn.com/v4/letter/a/a183cd/32.png) [@Allen\_Ingling](https://psychtoolbox.discourse.group/u/Allen_Ingling)\
**Post date:** [July 22, 2008, 1:04am UTC](https://psychtoolbox.discourse.group/t/timing-issue/1472/1 "2008-07-22T01:04:16Z")

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Hi group:  
  
I'm using psychtoolbox for simple fMRI experiments which mostly call  
on simple Screen commands. Recently, I received the following error:  
  
PTB-ERROR: Screen('Flip'); timestamping computed an \*impossible value\*  
1633.600234, which would indicate that stimulus onset happened  
\*before\* it was actually requested! Something is broken in your  
systems [sic] timstamping. High-precision timestamping disabled...  
  
  
There is some more to it, but that's the general idea.  
  
Since then, none of my experiments will function.   
  
Two questions:   
  
First, what can I do to fix this? Obviously, I have deleted old files  
and have reinstalled matlab and psychtoolbox.  
  
More importantly, how can I prevent this from happening again?