Since when? A while ago he convinced Dumbledore to give him the full six hours rather than two, but I don't think we were ever told that he can use it at will now.
ETA: From Chapter 77, Self-Actualization Aftermaths, emphasis mine:
(Some time later, an earlier version of Harry, who had invisibly waited next to the gargoyles since 9PM, followed the Deputy Headmistress through the opening that parted for her, stood quietly behind her on the turning stairs until they came to the top, and then, still under the Cloak, spun his Time-Turner thrice.)
I seem to have been mistaken. Still; Harry could simply destroy the protective measure.
EDIT: New discussion thread here.
This is a new thread to discuss Eliezer Yudkowsky's Harry Potter and the Methods of Rationality and anything related to it. With two chapters recently the previous thread has very quickly reached 500 comments. The latest chapter as of 17th March 2012 is Ch. 79.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author's Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven, eight, nine, ten.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: